Even in the Uber tier, there is no Pokémon who can do Blissey's job better. Although there are several Pokémon who can break through Blissey's Special Defenses in Ubers, don't let that discourage you from using Blissey, as it is still the best general special wall in the Ubers metagame. Almost any team will appreciate Blissey's ability to sponge special assaults, especially stall teams.
Even with incredible walls such as Giratina and Lugia in the Uber tier, Blissey is still the number one special wall to use. Blissey is the only Pokémon in Ubers that can wall such a wide variety of special threats, such as Latias, Latios (as long as it lacks Refresh or Safeguard), Palkia, Dialga (provided they aren't mixed), and Calm Mind Lugia. As far as attacking options go, Seismic Toss is generally the most reliable as it will do a decent amount of damage to Pokémon like Kyogre and Latias who would otherwise laugh at Ice Beam. Seismic Toss also allows Blissey to have a better chance of beating Mewtwo that have Calm Mind and Taunt, which Ice Beam can not accomplish. Ice Beam is still a good option though, as three common Blissey switch-ins (Groudon, Garchomp, and Rayquaza) are hit super effectively by it. With 84 SpA EVs, Garchomp takes a massive 76.19% - 90.76% from Ice Beam when it switches in, while a 0 HP / 4 SpD Rayquaza will be OHKOed after switching into Stealth Rock. In emergencies, you can leave Blissey in on Rayquaza, because the opponent may attempt to set up a Dragon Dance or Swords Dance. When using Seismic Toss, use the EV spread 4 HP / 252 Def / 252 SpD, as there is no reason to invest in Special Attack when using it. Toxic complements this set well, as the combination of Protect and Wish can very easily stall Pokémon to death. Toxic is a good choice to use if you predict the opponent is going to switch, as several threats to Blissey (Kyogre, Darkrai, and Giratina-O) become much easier to handle when badly poisoned. Thunder Wave can be used over Toxic, but make sure you use it only with Seismic Toss. This is because without Seismic Toss or Toxic, Latias can set up Calm Minds with no problem and eventually KO Blissey. Although not recommended due to moveslot competition, Heal Bell can be used over Protect if your team needs both a status reliever and a user of Wish.Team Options & Additional Comments >>>
This set is very similar to Wish Blissey, but it forgoes the supportive nature of Wish and Protect in exchange for Softboiled, which frees up an extra moveslot for Blissey. The best two moves for this moveslot are Ice Beam or Aromatherapy. With the EV spread above, Blissey's Ice Beam is able to OHKO Rayquaza with 4 Special Defense EVs 100% of the time after it takes Stealth Rock damage, thus making Blissey an effective Rayquaza lure.Team Options & Additional Comments >>>
With a few adjustments, Calm Mind Blissey can work very well in Ubers. Once again, Blissey should use a Calm Nature with an EV spread of 252 Def / 84 SpA / 172 SpD to OHKO Rayquaza with Ice Beam after it switches into Stealth Rock. Thunder should also be used in place of Thunderbolt, mainly due to its use with the rain support Kyogre gives Blissey, making Thunder 100% accurate, and a powerful attack that has a 30% chance to induce paralysis. While Serene Grace is most certainly a worthy option (especially with a 60% chance of paralysis with Thunder), Natural Cure is usually preferred because without it, Blissey will be crippled by Toxic.
One of the biggest selling points of Calm Mind Blissey in Ubers is the ability to beat certain threats that Wish Blissey and Aromatherapy Blissey would normally lose to. For example, Calm Mind Blissey can boost against Giratina-O and eventually destroy it with Ice Beam. Another good example is Choice Specs Kyogre. If Kyogre switches on Blissey as it uses Calm Mind, Kyogre will be 2HKOed by Thunder after Stealth Rock. Thus, Kyogre will no longer pose a threat to Blissey with its weakened Water Spout. It will also beat Calm Mind and Substitute Kyogre by Calm Minding along with it, while Blissey being able to easily recover its health and beat Kyogre with Thunder. Giratina that lack Calm Mind are also beaten by Calm Mind Blissey, because Giratina's Dragon Claw will only do 31% max to Blissey (even Outrage only does 46% maximum). However, Blissey requires three Calm Minds to 2HKO a 248 HP / 8 SpD Giratina. If Manaphy switches in on Blissey it will also lose. This is because Blissey can use Calm Mind as Manaphy sets up Tail Glow. Even a +6 Manaphy will be unable to 2HKO Blissey with Surf in the rain after Blissey has used Calm Mind three times. Meanwhile, a +3 Blissey will be able to OHKO Manaphy with Thunder. Substitute Mewtwo with Calm Mind and Latias or Latios with Calm Mind and Refresh or Safeguard will lose to Blissey in a way like Calm Mind Kyogre.Team Options & Additional Comments >>>
Blissey has a bundle of other support moves for use in Ubers. She is one of the few usable Pokémon in Ubers who has access to Stealth Rock. While Blissey is often better off using other support moves besides Stealth Rock, it is a viable move if no other Pokémon on your team can set it up. Light Screen may seem like an odd choice on Blissey, but it can be helpful for not only support the rest of your team, but will also make sure that Choice Specs Kyogre will no longer be able to 2HKO Blissey while Light Screen lasts. Blissey can use Sing to put would be counters to sleep, but note that Sing only has a 55% chance of hitting the opponent. Snatch can be used to steal Reflects, Light Screens, stat ups, and other support moves, but many users of such moves such as Mewtwo can easily use Taunt to shut down that strategy, in addition to Blissey often being better off using Calm Mind herself rather then stealing Calm Minds. Rest is almost always an inferior option to Softboiled or Wish, but it can work well with Natural Cure, allowing Blissey to be relativity unharmed by Toxic Spikes, and will allow Blissey to outstall opposing Blissey. Blissey can use Block to trap Wobbuffet switch-ins, and then use Toxic to slowly stall Wobbuffet to death. Copycat may seem like a strange option on Blissey, but it can be useful in certain situations. For example, it can allow Blissey to use Spikes or Toxic Spikes on Forretress as it uses them. If Forretress attempts to Rapid Spin, Blissey can either roast it with Flamethrower (provided that its sunny), or Copycat Rapid Spin to get rid of the opponents entry hazards. Copycat can also allow Blissey to defeat Calm Mind Giratina-O and Calm Mind Kyogre that lack Rest. Note, however, that Copycat is illegal with Seismic Toss and Wish.
Charm can be used to soften physical attackers attempting to prey on Blissey after a switch-in. Due to Charm, mixed Palkia (barring a Focus Punch) and Dialga will no longer be able to get past Blissey beyond scoring a critical hit, while Groudon and Garchomp will only have a small chance to 2HKO Blissey with a Choice Banded Earthquake and Outrage respectively. Note that both Garchomp and Groudon have access to Swords Dance, thus it is best to still have Pokémon who can defeat both. While an Adamant Life Orb Rayquaza can still 2HKO Blissey with Outrage after being hit by Charm, she can OHKO Rayquaza with Ice Beam after it switches into Stealth Rock and Charm (provided she has 84 SpA EVs), allowing Blissey to defeat a Pokémon by whom she is normally greatly troubled. Charm also has the potential to cause a large amount of switches, thus it is best to pair Charm Blissey with users of entry hazards. Note that both Wish and Seismic Toss are illegal to use with Charm.
Blissey has access to Gravity. Gravity will allow both Groudon and Garchomp to freely use Earthquake and a 100% accurate Stone Edge, as Gravity eliminates the immunity Flying-type Pokémon and Pokémon with the Levitate ability for five turns. Gravity will also eliminate the immunity floating Pokémon have to Spikes and Toxic Spikes, meaning that Gravity Blissey will greatly benefit Forretress and Skarmory's Spikes (and Toxic Spikes in Forretress' case) laying abilities. Finally, Gravity will lower the evasion of all Pokémon by 2 stages, meaning that any move that has at least 60% accuracy will always hit. This is especially great for Darkrai, who must often use the 70% accurate Focus Blast and the 80% accurate Dark Void. Calm Mind Mewtwo with Life Orb will also enjoy the effects of Gravity, as it allows Life Orb Mewtwo to safely use Focus Blast, which will always 2HKO opposing Blissey after one use of Calm Mind. Garchomp not only benefits from an Earthquake that hits all Pokémon, it also enjoys the evasion cut as well. This is because the usually unreliable Dragon Rush becomes viable on Garchomp under Gravity, meaning that it will have access to a STAB 100 Base Power Dragon-type attack that has a 20% chance to flinch. Blissey's Sing will also hit 92% of the time under Gravity. There is one potentially fatal flaw with Gravity, and it is that the opponent can also take advantage of Gravity's effects. This will make Groudon and Garchomp particularly difficult to handle, thus it is best to either destroy them before using Gravity.
Blissey has some alternate attacks at its disposal that can be useful in certain situations. When under the support of Groudon's everlasting sun, Flamethrower becomes a viable option to KO or cripple several common Steel-types that may switch into Blissey. Forretress is always OHKOed by Flamethrower after Stealth Rock, while only extremely specially defensive Scizor will survive Flamethrower after Stealth Rock damage. Blissey's Flamethrower will do a massive 76% - 90% to Lucario in the sun, while Metagross is always 2HKOed after Stealth Rock damage. Grass Knot may normally be an inferior option to Thunder on Calm Mind , but it has two main selling points. Grass Knot is always a 100% accurate attack, regardless of the weather, which is particularly useful considering the fact that many Ubers are heavy enough to be hit by a 120 Base Power Grass Knot. Secondly, Groudon is hit quite hard by it. Finally, Counter can be a good option on Blissey, even in Ubers. Several Pokémon, such as Palkia and Dialga, will use mixed sets just to defeat Blissey. If Blissey predicts a physical attack from either of them, and uses Counter, Palkia and Dialga are sure to be KOed. Also, note that defensive Groudon, Choice Scarf Garchomp, and Scizor that lack Superpower are also unable to OHKO Blissey, meaning that Blissey will be able to gain a surprise kill on them if she is healthy.
In the Uber tier, Blissey should use 4 HP / 252 Def / 252 SpD with a Calm nature on all sets without Ice Beam, as Blissey needs to be extra durable to take the powerful assaults of Ubers. If using Ice Beam, use the EV spread of 252 Def / 84 SpA / 172 SpD, because this will allow Blissey to OHKO a 0 HP / 4 SpD Rayquaza 100% of the time after it switches into Stealth Rock.
Checks and Counters
In the Uber tier, Bulk Up Dialga is, without a doubt, the best Blissey counter. It is immune to Toxic and can easily set up on Blissey. Powerful physical attackers such as Garchomp, Groudon, and Rayquaza can make Blissey's life a living hell. However, Ice Beam will do massive damage to Garchomp and Rayquaza and none of them will like being hit by Toxic.
Many Pokémon in the Uber tier can surprise Blissey with mixed attacking sets, such as Mewtwo, Darkrai, Dialga, Palkia and Giratina-O. Dialga in particular can be a problem, as it is immune to Toxic.
If Blissey lacks Calm Mind or Psych Up, Kyogre and Giratina can use a combination of Sleep Talk, Rest, and Calm Mind to easily set up on Blissey. Kyogre, Mewtwo, and Giratina-O can also use Substitute and Calm Mind to beat her. In the rain, Manaphy can easily set up Tail Glow on non-Calm Mind Blissey and 2HKO it with a +2 Surf. Choice Specs Kyogre also has the capability of 2HKOing Blissey with Water Spout.
Wobbuffet can switch into Blissey and use Tickle several times to lower its Defense. After this, Blissey will be KOed by Pursuit from Scizor or Tyranitar. Wobbuffet can also use Encore on Blissey and switch in a sweeper such as Lucario or Giratina-O to take advantage of the free set up time.