I have had much better success with this Blissey than the Seismic Toss variants:
Blissey @ Leftovers
Ability: Natural Cure
EVs: 252 HP/ 4 SpA / 252 Def
Nature: Bold/Calm
- Aromatherapy/Toxic
- Ice Beam
- Fire Blast/Flamethrower
- Softboiled
Normally Blissey would have Seismic Toss and one or no other coverage move like Ice Beam or Flamethrower. But I have found Seismic Toss to just be set up bait for Salamence/Scizor depending on the coverage move that it lacks. I find myself switching much more often than needed. With both Ice Beam and Fire Blast, I find myself luring Scizor switches like no tomorrow, each one OHKOed/ 2HKOed (96.8% - 114.3%) by a easily predicted Fire Blast (guaranteed OHKO for standard CB Scizor in Stealth Rock with Fire Blast), and providing much better momentum. With Ice Beam and Fire Blast, some of OUs most common Blissey set ups do not hinder your momentum. To list a couple: Salamence, Scizor, Gliscor, Dragonite, Forretress, Donphan, Ferrothorn, Skarmory, Landorus, Tornadus, Thundurus, Hydreigon, Breloom, Dugtrio, Lucario, Magnezone, Metagross. I'm not saying at all that Blissey can beat all of these one-on-one (some of them it can), but I'm saying that Ice Beam and Fire Blast have much better coverage and makes opponents more wary of setting up on Blissey than Seismic Toss could ever do. One example might be Thundurus. Without Ice Beam, he would set up all over you, and provided that you did have Ice Beam (2HKO), which after the first hit would simply force the opponent to swap in his Scizor, and now you have to switch loosing the momentum. Where as if you had Fire Blast, a really easily predicted Scizor switch in would allow for an easy OHKO or 2HKO on the switch. Now you might still have to switch, but now your priority attackers or your pursuit Scizor can easily trap and give you the momentum. In addition this heavily differentiates Blissey from Evolite Chansey, which would be set up bait to all of the aforementioned mons and cannot even 2HKO anything with its garbage Special attack. To conclude, I am only saying that Ice Beam and Fire Blast/Flamethrower are much more useful on the current OU threats, do much more damage than standard Seismic Toss and make them wary of setting up on Blissey.
EDIT: As ScarfWynaut points out, Calm nature might be better suited for more special bulk, against the common hard hitting special mons around in today's metagame. And the drop in defense isn't too significant. I usually need Aromatherapy support, and tend to have a Politoed or Volcarona counter/check, but if you don't need Aromatherapy, it can be slashed for Toxic to deal with special threats.