Power up Blissey?

depending on things (area like how many people play active and how many people put normal type tanks in gyms, do you have enough candies not worth inversting rare candies and last but not least do you have a lot of star dust i would say under 400k dust dont up go for it)

Unless your gym scene is very slow, it makes sense to power it up. If you drop a pidgey in a gym and it sits there for three days, then there's no reason to use Blissey for that - it will actually be worse, losing motivation faster (and even faster if powered up). But if you're fighting to stay in a gym for eight hours, Blissey is the best option.

I believe your need for you to power it up will come in time.

The answer to your question as of now would be, No. Because otherwise, you wouldn't be asking and instead would be maxing that blissey.

It depends on how active your area is, if you can stay in a gym for 8 hours or go overnight unchallenged then you're good.

Without knowing your particular situation (gym scene, dust, other priorities), it’s hard to say.

However, if the time is right for you to power up a Blissey, that one is a great candidate. Best defensive moves, perfect attack, and overall stamina stat that is 99.4%

Probably no.

I have a Blissey over 3000 CP that I powered before the stat rebalance was announced. It does well in gyms, but if someone wants a gym and I don't Golden Razz it (and sometimes even if I do) they'll take it. Further, at that high of CP it decays pretty rapidly.

I think as is, your Blissey will decay much slower. So it will do a lot better at keeping a gym for a longer time. With the upcoming stat rebalance being a big nerf to Blissey, I'd not power. Focus on attackers. At least wait until PvP to consider tdo options.