Blissey power-up or evolve

I'd do neither and hunt for a high level Chansey with better IV's. That's a good Blissey, but you are going to spend a ton of candy powering it up. A week ago I found a 93% Chansey, level 29, so keep hunting those Chanseys!

what are the exact hp-at-df IV of your Chansey?
level is pretty high...

If Att=15, you can run calcs here to see what the CP would be for the 1166 Chansey compared to max CP for perfect IVs.

Assuming it's not 15 Att, I'd probably do neither and keep looking due to Blissey being 6th on CP rankings (I get shaved off in my area) and just save the candy.

I agree on the "neither", but then again it is very VERY hard to get a high level Chansey with good IVs.

Gun to your head...if you have to do one, I would personally do the higher level, lower CP. you will immediately have a great gym defender who will be GREAT if you power up a few times. Then you still have around 100 candies left, and you wait. Realistically, the best chance to get a higher level, high IV chansey is to hatch him, and if you do then you'll be ready to max that guy out. The chances of catching one are very low (considering how hard it is to catch one in the first place)

I know my opinion won't be popular since we all (me included) are obsessed with IVs, but a 58% Blissey at level 30 is still a BEAST, with only small CP difference to a high IV, and a tiny performance differencce.

Also, this is just a small thing... but on your new Blissey he doesn't have the best moveset, so you can also hope to improve there.

Good luck

For me, a lot would depend on the IV spread on the 58% Chansey. HP IVs aren't worth much on it given it's incredibly high base stamina, while Att and Def IVs are extremely valuable. If the 58% was comprised solely of Att and Def I would certainly consider evolving it.