Is this Blissey viable for first-line gym defense?

Sure. It will show up as a 2309 Blissey the same as any 2309 Blissey. It will decay less fast than a 2800 or 3000 or ... Blissey. With ZH/P it will punish attacking fighting types.

I stopped powering my 2 Blisseys at 2400 and 2200. They defend just fine.

I'm actually getting concerned about potions, maybe you are, too. I'd put it in a gym...have it return---then try to leave it unhealthed. If potions become common again use it as your backup's backup (maybe add another few backups to that)

You caught a wild Blissey? Wow.

Those are some hilarious IVs, but it will serve fine. That's probably the second best moveset on defense.