PowerUp a 75% Blastoise with a perfect moveset?

If you are looking for a competitive pkmn (especially attacker)...Evolve high level and 80+ IV Eevee, and if you get Vaporeon you have a stronger pkmn than Blastoise. For just 25 Eevee candies!

Actually waiting for better IV. But as you say you envolve already at 80% you have to consider your stardust situation. If u have enough and there is nothing else to power up go ahead. but dont expect to much. Vaporeon with wg/hp would be much cheaper in most of the cases.

Smart move is vaporeon and not touch blastoise.

fun/dumb move would be power up blastoise.

Blastoise looks utterly badass and no one uses him because of the prevalence of the op and easily attainable vaporeon.

Evolve an 80+ iv evee because it easier to get, have a better cp.

care, dont wasted your stardust.
up 1 or 2 pokemon "fun" maximum
(exemple gengar for me)
and safe the rest for optimal pokemon (exemple vaporeon)
vaporeon is very "potion effective" for batteling.
"CP / dust effective" too

To answer your question Yes I would Power up your Blastoise.

In your situation I rather have a Blastoise with a desired move set and decent IV's than a Blastoise with perfect IVs but has a crappy move set.
For example I waited patiently and evolved Magikarp to Gyarados with perfect IVs but got Bite/Twister- I Never use it.
However, I have a Venusaur that I caught in the wild and is above average but has Vine Whip/Solar Beam!! I couldn't be anymore happier. I power him up and use it all the time.

It comes down to what is going to make you have the most enjoyment out of the game.

980CP with Water gun/Hydro pump is perfect for training against ~2000CP Arcanines. I have a Wartortle with the same moveset at about that CP that I constantly use.

I would wait for something better to power up to use for attacking enemy gyms.