Which Blissey to power up? (To lev 31 for now)

I found a website: www.pokebattler.com

B without question. Best defensive Set, great IV, you will not regret anything. ;)

I would max your lvl 31 if not already and then A so as to get max CP

Blissey will not have a bad moveset simply due to its monster stats.

However I would probably rate def stats like so:

A - P + DG
A - ZH + DG
A - P + HB
A - ZH + HB
B - P + PSY
B - ZH + PSY

A, despite lower IV, will have 44 more CP than B at level 31 : 2768 vs 2724
Att stat makes a huge difference for Blissey.

I would evolve A and power up if no psychic.

Definitely the one with 15 to ATK IV. That IV is super important to Blissey, I almost wouldn't even evolve one without 14-15 in attack because of the huge effect it has on Blissey's max CP. any gains in HP are negligible, the least important IV by far.