Blissey isn't that good as a special attacker, but neither is Clefable(only one f). Stat-wise, Clefable has a much better special attack. The key difference is: Clefable learns Stored Power, and Blissey does not. Both are tanky, you do not need any speed investments. Blissey is more of a status farmer, Clefable is a tank and an attacker(with some sets).
Blissey's only good special setup move(which it needs unless you want to go status farming without choice specs) is calm mind, which its special defense doesn't really benefit from unless your opponent has a x3 or higher special attacker, which is very rare. It can already tank any non-boosted special attack(except for Psyshock), so Blissey doesn't have much point in running special. However, here's a set anyways.
Blissey (F) @ Leftovers
Ability: Serene Grace
EVs: 4 HP / 252 SpA / 252 SpD
Modest Nature
IVs: 0 Atk
- Calm Mind
- Thunderbolt
- Flamethrower
- Ice Beam
Healer is useless because it's not doubles, you don't want to switch out so natural cure is useless, so serene grace is the only one to go. It's actually pretty good, because now it's a 20% chance to get the status.
Blissey (F) @ Choice Specs
Ability: Natural Cure
EVs: 4 HP / 252 SpA / 252 SpD
Modest Nature
IVs: 0 Atk
- Flamethrower
- Ice Beam
- Psychic/Tri Attack
- Thunderbolt
For this set, you want to be switching out. Use Natural cure to take care of paralysis, burn, and paralysis. Switch into a special attacker and get some damage(and hopefully status), then switch if your move is not good for the current opponent.
With Clefable's okay special attack stat, just have cosmic power with stored power and you're good. Unaware is a very good ability anyways, so Clefable can be a really strong special attacker as well as an unkillable tank. Or you could run magic guard life orb.
Clefable @ Life Orb
Ability: Magic Guard
EVs: 4 HP / 252 Def / 252 SpA
Modest Nature
IVs: 0 Atk
- Moonblast
- Thunderbolt
- Flamethrower
- Psychic
Mostly neutral attack power, not as good as the other set in my opinion. Still decent in some situations. This set likes to stay in and get chip damage when your opponent switches.
Clefable @ Leftovers
Ability: Unaware
EVs: 4 HP / 252 Def / 252 SpA
Modest Nature
IVs: 0 Atk
- Cosmic Power
- Stored Power
- Soft-Boiled
- Fire Blast/Thunder/Moonblast/Blizzard/Shadow Ball
The best set in my opinion, you want damage rather than accuracy. You can already live long with unaware leftovers cosmic power, so it's okay to miss a few moves. If you want to run life orb magic guard, use the weaker but more accuracy moves(flamethrower, thunderbolt, ice beam). Never switch, just cosmic power, soft boiled, repeat until you have at least x2. Try to get maxed out defences, then just win(if your opponent has not yet ragequit, that is).
There you have it! My personal opinion and a few sets. Clefable is better in my opinion, but Blissey is always okay. Always run 0 ivs in attack with an attack lowering nature so that foul play does less damage.
Hope I helped!