Blissey in no way "needs" Thunderbolt, much like it doesn't "need" Ice Beam. Seismic Toss is standard, as is Softboiled, and any reasonable Blissey user knows that Aromatherapy/Heal Bell works wonders, especially on a stall team (where Blissey normally works the best) where you have limited offensive power.
Here's the standard for a Blissey:
Blissey @ Leftovers
Trait: Natural Cure
Bold Nature
EVs: 252 HP, 252 Def, 6 SpDef/SpAtk
- Seismic Toss
- Aromatherapy
- Softboiled
- Ice Beam/Thunder Wave
Only go with Ice Beam if you really fear going head to head with a Ghost, because besides that it won't help you much. It wouldn't kill Garchomp or Salamence and doesn't OHKO Dugtrio. Thunder Wave is a lot more helpful, as it's a team support move. And because she has Natural Cure, she's safe to paralyze the likes of Alakazam, who is one of the only Special Sweepers who can beat Blissey one-on-one. But if you do decide to go with Ice Beam, put those 6 EVs into SpAtk. If you choose Thunder Wave, put them into SpDef.
If you really want Thunderbolt, you could go with a rare yet formidable BoltBeam Blissey. Requires a bit of reworking, but can be effective:
Blissey @ Leftovers
EVs: 252 HP OR Special Attack, 252 Def, 6 HP OR Special Attack
- Softboiled
- Ice Beam
- Thunderbolt
- Calm Mind