I think I got prescribed to the wrong drug.... some Special Attacker named Keldeo got past Blissey!
*The metagame has filled up with too many strong attackers and multiple methods of getting past Blissey for her to remain a top-tier threat.
*The popularity of Keldeo in particular is too much for Blissey to handle. Strong Psychics, like Latios and Alakazam, also have an easier time with Blissey with their access to Psyshock.
*Faces competition with her younger sister, Chansey, who was blessed with the Eviolite, which allows her to take some more physical hits, as well as special ones
*Unlike Chansey, Blissey can hold Leftovers, allowing it to negate the residual damage from Sand, Hail, and Entry hazards.
*Her HP is the highest of any Pokemon, and can make use of it with massive Wishes that can heal more than 300 HP
*Faces little competition as a cleric
*Needs some support more than ever to cover up her weaknesses, and is setup bait for some dangerous physical sweepers, like Gyarados, Lucario, and Landorus-T.
move 1: Wish / Softboiled
move 2: Protect / Aromatherapy
move 3: Toxic
move 4: Seismic Toss / Flamethrower
ability: Natural Cure
evs: 4 HP / 252 Def / 252 SDef
*Blissey's monstrous base 255 HP stat is ideal for passing Wishes to its teammates, as it restores a ton of HP to a plethora of Pokemon.
*Softboiled reduces Blissey's reliance on Wish for recovery
*Protect should only be used with Wish, as it allows Blissey to receive the Wish and scout the opponent
*Aromatherapy prevents Blissey from being Toxic-stalled. It also wakes up a sleeping teammate, who would otherwise be out for the entire match
*Softboiled + Aromatherapy, and Wish + Protect go hand-in-hand together.
*Toxic allows Blissey to wear down the opponent, and is Blissey's main form of damaging the opponent, and works well with Protect.
*Seismic Toss prevents Blissey from becoming complete setup bait by sweepers (especially most Substitute users trying to avoid Toxic), and allows it to reliably damage the opposition without worrying about its low offenses.
*Flamethrower hits most Steels immune to Toxic hard, such as Forretress, Scizor, and Ferrothorn, as well as making Blissey a better counter to Venusaur. However, it fails to guarantee a 2HKO on Ferrothorn.
*EVs maximize defenses, and Calm Nature is used to allow her to wall Special Attackers better.
*Wish can be paired with Softboiled to reduce Blissey's dependence on receiving Wish. Aromatherapy should always be used with Softboiled. However, if Blissey needs neither Wish or Aromatherapy, Flamethrower can be used in the second slot in tandem with Seismic Toss.
*Fire Blast can be used in place of Flamethrower to 2HKO Skarmory and Ferrothorn, but its measly amount of PP is an issue for a defensive Pokemon
*Thunder Wave cripples some of the Pokemon that tend to switch into Blissey, such as Jirachi, Scizor, Terrakion, Salamence, etc. Thunder Wave also allows Blissey to cripple Venusaur.
*Blissey functions well on stall and other defensive teams who appreciate the utility of Wish and can cover Fighting-types and physical attackers in return. Jellicent, Latias, and Skarmory are good examples of such.
*Entry hazards are annoying for a wall to have to stomach. Tentacruel and Forretress can Rapid Spin, abuse Blissey's Wishes, and lay down hazards of her own. Ferrothorn can check Dragon Dance users, as well as Latios and Kyurem-B. Landorus-T checks all sorts of physical attackers not named Mamoswine, Weavile, or Kyurem-B
*Blissey is best used in Sand or Hail, where Chansey gets worn too easily, so Hippowdon and Abomasnow are good teammates. Tyranitar is not bad either, but it increases the weakness of Fighting-types to the team by a very large margin.
*Stealth Rock is a move that literally every team must have, but there are so many Pokemon that learn it, so Blissey should rarely ever need it.
*Blissey also has Thunderbolt, but she really does not have the space for them, and they are pitifully weak for such a low uninvested Special Attack. However, Ice Beam can hit several Dragons hard, such as Garchomp and Salamence.
*Blissey can use Calm Mind to boost her high Special Defense and her mediocre Special Attack, but she is still weak to physical attacks, such as from Scizor and Breloom.
*Light Screen, but that is better left to Pokemon like Cresselia or Bronzong, who have access to Reflect as well
*Serene Grace increases the chances of secondary effects, like Ice Beam's chance to freeze, but Natural Cure is generally the better option, because Blissey generally does not stay in for too many turns. Natural Cure also allows Blissey to recover from Poison and Burns, which would otherwise cripple her.
*Counter and Gravity are less orthodox options, but can work, due to Blissey's insane HP stat
*Skill Swap can be used to stop Reuniclus from destroying your team, but Blissey is hit hard by Psyshock.
*Blissey has some gimmicky options in Defense Curl, Sing, and Charm, but Blissey has better things to do.
[CHECKS AND COUNTERS]
*The large list of Fighting types in OU are the best way to keep Blissey in check; Terrakion, Breloom, Keldeo, Infernape, Conkeldurr, Lucario, etc.
*Gengar shuts down any Blissey set, and can Taunt her or use Substitute + Pain Split to wear her down, or Substitute + Disable to keep her from passing Wishes.
*Any physical attacker with a boosting move or a Fighting-type move can muscle past Blissey
*Jirachi is immune to Toxic, and can set up 101 HP Substitutes to allow it to get multiple Calm Minds in without getting worn down by Seismic Toss. It can also paralyze Blissey and flinch it down with Iron Head, or trick a Choice Scarf onto it.
*Other Trick users, like Latios and Rotom-W, cripple Blissey by replacing her beloved Leftovers with a handicap.
*Despite having Natural Cure, Toxic Spikes annoy Blissey a lot, as they wear her down rather quickly