http://www.smogon.com/dp/pokemon/blissey
I question the use of Seismic Toss Blissey in DP. Sure, not using it makes you weaker to, say, Calm Mind Substitute Raikou, but you can use Tyranitar or Hippowdon or several other Pokemon to take that on. Compare that to the threat of Gengar, which is much larger. Gengar alone is usually the reason I stick Blissey on my team, as opposed to just relying on something like Tyranitar + Celebi. For this reason, I would stick Flamethrower, Thunderbolt, or Ice Beam as the primary attack of every set, with Seismic Toss as a secondary option, if it's on the set at all.
Moreover, I feel that Blissey needs Special Defense EVs (and a Calm nature) to wall the special threats that do the most damage (in other words, the real threats). You can wall Milotic and Zapdos all day with Blissey regardless of your EV spread, yes, but I've never been straight-up swept by either of those. Things like Lucario and Gengar, however, are much more dangerous, and these require some EVs to take on.
For that reason, this is my update to each set. I'm leaving out the set descriptions for now because I have to rewrite a bit of a lot of them, and I don't want the way I say something to get in the way of what I am saying. What comments I write are also dependent on the final set, so I don't want to write something only to have to rewrite it.
[SET]
name: WishBliss
move 1: Wish
move 2: Protect / Softboiled
move 3: Flamethrower / Ice Beam
move 4: Toxic
item: Leftovers
ability: Natural Cure
nature: Calm
evs: 252 Def / 80 SpA / 176 SpD
Any attack can go over Toxic (Ice Beam, Thunderbolt, Hidden Power Ground, and Seismic Toss are the main moves in the category of 'any attack' unless you need to beat something specific, like Grass Knot for Swampert). I strongly recommend Flamethrower as the lone attack if you use Toxic. This can survive a CH Specs max damage Modest Lucario Aura Sphere if you are at full health, letting you Wish and Protect the next turn.
[SET]
name: Cleric
move 1: Aromatherapy
move 2: Ice Beam
move 3: Thunder Wave / Toxic / Seismic Toss
move 4: Softboiled
item: Leftovers
ability: Natural Cure
nature: Calm
evs: 252 Def / 40 SpA / 216 SpD
Slightly more special defensive version because you can't stall out stuff with Protect. Just to show the max SpD I'd use on Blissey.
Any special attack can go in slot 2. You can also use another special attack in slot 3, but I prefer status.
[SET]
name: Special Wall
move 1: Ice Beam
move 2: Seismic Toss
move 3: Sing / Thunder Wave / Toxic
move 4: Softboiled
item: Leftovers
ability: Natural Cure
nature: Bold
evs: 148 HP / 252 Def / 104 SpA / 4 Spe
I'll keep the older EV spread on the 'older' set.
[SET]
name: CMBliss
move 1: Calm Mind
move 2: Ice Beam
move 3: Thunderbolt
move 4: Softboiled
item: Leftovers
nature: Bold / Modest
evs: 4 HP / 252 Def / 252 SpA
Max Special Attack is needed to guarantee the KO on 4 HP Chomp. It also gives you a good shot at OHKOing min HP Gyarados. I'm going to mention the ability to go more defensive with HP EVs. I don't think Calm Mind Blissey really has as much need (or room) to use Special Defense EVs.
You can use several combinations of Special Attacks. My new favorite is probably Flamethrower and Hidden Power Ground. Those two moves catch the most common Blissey switch-ins SE. It's nice to show Flamethrower, and then Calm Mind up once or so and nail Heatran for a OHKO. If you use that, you'll need Serene Grace, because your only option to really hurt Garchomp in particular is to burn it. 20% burn is good enough, I'd say (you'll obviously be wanting to open with Flamethrower to try and burn stuff as well as catching guys like Lucario and Heracross). I don't like Grass Knot with Flamethrower as much as I once did (it didn't pan out in testing). I'd rather just use Thunderbolt to cover Gyarados and Heatran better. You don't have a chance to Calm Mind up enough to do any serious damage to Tyranitar, anyway, so you might as well try and burn it with Flamethrower instead.
[SET]
name: CharmBliss
move 1: Charm
move 2: Ice Beam
move 3: Light Screen
move 4: Softboiled
item: Leftovers
ability: Natural Cure
nature: Calm
evs: 252 Def / 120 SpA / 136 SpD
Charm Blissey has even less of a need for HP EVs, I'd say.
[EVs]
Stuff about how aside from the Calm Mind set, any EV spread can be used on any set, with preference given to Special Defensive spreads.
I question the use of Seismic Toss Blissey in DP. Sure, not using it makes you weaker to, say, Calm Mind Substitute Raikou, but you can use Tyranitar or Hippowdon or several other Pokemon to take that on. Compare that to the threat of Gengar, which is much larger. Gengar alone is usually the reason I stick Blissey on my team, as opposed to just relying on something like Tyranitar + Celebi. For this reason, I would stick Flamethrower, Thunderbolt, or Ice Beam as the primary attack of every set, with Seismic Toss as a secondary option, if it's on the set at all.
Moreover, I feel that Blissey needs Special Defense EVs (and a Calm nature) to wall the special threats that do the most damage (in other words, the real threats). You can wall Milotic and Zapdos all day with Blissey regardless of your EV spread, yes, but I've never been straight-up swept by either of those. Things like Lucario and Gengar, however, are much more dangerous, and these require some EVs to take on.
For that reason, this is my update to each set. I'm leaving out the set descriptions for now because I have to rewrite a bit of a lot of them, and I don't want the way I say something to get in the way of what I am saying. What comments I write are also dependent on the final set, so I don't want to write something only to have to rewrite it.
[SET]
name: WishBliss
move 1: Wish
move 2: Protect / Softboiled
move 3: Flamethrower / Ice Beam
move 4: Toxic
item: Leftovers
ability: Natural Cure
nature: Calm
evs: 252 Def / 80 SpA / 176 SpD
Any attack can go over Toxic (Ice Beam, Thunderbolt, Hidden Power Ground, and Seismic Toss are the main moves in the category of 'any attack' unless you need to beat something specific, like Grass Knot for Swampert). I strongly recommend Flamethrower as the lone attack if you use Toxic. This can survive a CH Specs max damage Modest Lucario Aura Sphere if you are at full health, letting you Wish and Protect the next turn.
[SET]
name: Cleric
move 1: Aromatherapy
move 2: Ice Beam
move 3: Thunder Wave / Toxic / Seismic Toss
move 4: Softboiled
item: Leftovers
ability: Natural Cure
nature: Calm
evs: 252 Def / 40 SpA / 216 SpD
Slightly more special defensive version because you can't stall out stuff with Protect. Just to show the max SpD I'd use on Blissey.
Any special attack can go in slot 2. You can also use another special attack in slot 3, but I prefer status.
[SET]
name: Special Wall
move 1: Ice Beam
move 2: Seismic Toss
move 3: Sing / Thunder Wave / Toxic
move 4: Softboiled
item: Leftovers
ability: Natural Cure
nature: Bold
evs: 148 HP / 252 Def / 104 SpA / 4 Spe
I'll keep the older EV spread on the 'older' set.
[SET]
name: CMBliss
move 1: Calm Mind
move 2: Ice Beam
move 3: Thunderbolt
move 4: Softboiled
item: Leftovers
nature: Bold / Modest
evs: 4 HP / 252 Def / 252 SpA
Max Special Attack is needed to guarantee the KO on 4 HP Chomp. It also gives you a good shot at OHKOing min HP Gyarados. I'm going to mention the ability to go more defensive with HP EVs. I don't think Calm Mind Blissey really has as much need (or room) to use Special Defense EVs.
You can use several combinations of Special Attacks. My new favorite is probably Flamethrower and Hidden Power Ground. Those two moves catch the most common Blissey switch-ins SE. It's nice to show Flamethrower, and then Calm Mind up once or so and nail Heatran for a OHKO. If you use that, you'll need Serene Grace, because your only option to really hurt Garchomp in particular is to burn it. 20% burn is good enough, I'd say (you'll obviously be wanting to open with Flamethrower to try and burn stuff as well as catching guys like Lucario and Heracross). I don't like Grass Knot with Flamethrower as much as I once did (it didn't pan out in testing). I'd rather just use Thunderbolt to cover Gyarados and Heatran better. You don't have a chance to Calm Mind up enough to do any serious damage to Tyranitar, anyway, so you might as well try and burn it with Flamethrower instead.
[SET]
name: CharmBliss
move 1: Charm
move 2: Ice Beam
move 3: Light Screen
move 4: Softboiled
item: Leftovers
ability: Natural Cure
nature: Calm
evs: 252 Def / 120 SpA / 136 SpD
Charm Blissey has even less of a need for HP EVs, I'd say.
[EVs]
Stuff about how aside from the Calm Mind set, any EV spread can be used on any set, with preference given to Special Defensive spreads.