Wild blissey?

Many on here will suggest waiting until you can max her out in one shot, but I think it depends on how much you need a defender and what level it would take to hold a spot in your local gym scene. I have one Blissey maxed, but my second one is only at the CP 1811 when she evolved. I had no more candy at the time. After 3 lucky Chansey hatches, I now have enough to max her out, but I can't because I stuck her in the bottom spot of a stable gym. Remarkably, she's held on for over a month without getting shaved, and earned me over 300 coins. I think if you can get her to a useful level, why not go ahead?

I would wait until I had enough candies to power her up to Level 30.

UncleJohnsHand may have had a 1800 Blissey in a gym for a month, but where I live it wouldn't last a day. Maybe if you stick in the last slot in a level 10, but even then it will only last a few days.

Any other level gym, and you prevent your teammates from leveling up the gym by sticking a low CP Blissey in there.

And most of all, you never know what will happen. Maybe you catch another wild Blissey that is better, or hatch a couple chanseys. Or maybe Niantic nerfs Blissey.

You may choose to use it as an attacker or a prestiger...so perhaps feeding her as you get candy might be an idea.
If it is solely going to be a defender, then it is determined by your local gym scene.

Congrats!

Keep it as a prestige, throw it away, or keep it as a trophy.

You can hatch/catch a better Chansey, evolve it and power it up higher. This Blissey will take a huge investment of dust and candy for such a low IV and not a good moveset. Idealy you want ZH/DG or ZH/HB for defense.