*Blissey is my favorite Pokémon... whooooo*
For me, I don't want Aromatherapy or Seismic Toss on Blissey anymore. Seismic Toss Blissey is so hard to get; I think the only way to do so is to migrate it from GBA. On the other hand, Aromatherapy needs you to make a Lv. 50+ male Roselia. Waste of time...
Wait! Doesn't Heal Bell do the same thing as Aromatherapy? The best way is to get a male Skitty (it learns Heal Bell pretty early), but even that may be hard.
No, I decided to remove them altogether. Based on the ever-helpful Serebii.net to which I am eternally grateful, I decided to pursue this set:
Blissey @Leftovers
Ability: Natural Cure
Nature: Bold
EV's: 4 HP/252 Def/252 SA
Sing/Calm Mind
Softboiled
Thunderbolt
Ice Beam
I know there's Sleep Clause, but Sing is still very essential to take out physical sweepers like Garchomp and Rhyperior. Despite the 55% accuracy, it can be remedied with either Wide Lens or Zoom Lens, though it's not worth the Leftovers sacrifice. Bolt-beam is resisted by only a few Pokémon, but other Special Walls will laugh at Blissey for it. That's why there's the possibility of Calm Mind. Keep Bliss healthy and use 6 Calm Minds, and you're off.
Another set is:
Blissey @Leftovers
Ability: Serene Grace
Nature: Modest
EV's: 4 HP/252 Def/252 SA
Sing
Softboiled
Ice Beam
Charge Beam
Charge Beam may not be very good in terms of base damage, but it has a 70% chance of raising Special Attack, which makes it ALL THE TIME with Serene Grace. With a maximum of 273 SA at Modest and 31 IV's, this Blissey will also be a force to be reckoned with.