Standard Blissey is [252 HP, 252 Def, 6 SpDef/SpAtk].
However, it depends on her moveset too. The standard usually applies to:
- Seismic Toss/Counter
- Softboiled
- Aromatherapy/Heal Bell
- Thunder Wave/Ice Beam
6 in Special Defense if you run Seismic Toss Blissey without Ice Beam or Thunderbolt or the likes (perhaps using Thunder Wave over it), but if you run Ice Beam Blissey, the 6 can go into Special Attack. Trivial, really, but I prefer to max out both HP and Defense.
If you go with a BoltBeaming Blissey, or a Calm Mind Blissey, then it's best to put some focus on Special Attack.