I started with Blissey but have since moved to Eviolite Chansey as a waller. That said, here's a fun Serene Grace Blissey I made that can really wreck people's day with a little luck:
Nature: Bold (+Defense -Attack)
Ability: Serene Grace (Damaging moves that can have a secondary effect on the target have a doubled chance of inflicting that effect).
EV Stats: +252 Defense, +152 Spec. Defense, + 106 HP improvable?
Held Item: King's Rock (All damaging moves have a 20% chance of flinching the opponent).
Moves: Charge Beam (Damage + 140% chance of boosting Spec. Attack), Fire Blast (Damage + 60% chance of inflicting burn), Ice Beam (Damage + 20% chance of freezing), Soft-Boiled (Heals 50% of HP).
This Blissey retains her defensive nature while being able to do effective special damage to most Pokemon types, with each offensive move also coming with a good chance of a secondary effect (Flinch, Burn, Frozen, guaranteed Spec. Attack boost). Burn is excellent as it lower's the opponent's attack stat, making Blissey much harder to kill.
Alternatives: Sweet Kiss over Ice Beam, Calm Mind over Charge Beam, Leftovers over King's Rock.
EDIT: After using this Blissey in a good number of battles now I have to say it's not bad at all, in fact I've been using it over Chansey pretty often as a rating battle sp. def. choice. If you can get just two Charge Beams off she really becomes a great attacker.