If you buy fish for color, just get a painting. LOL sorry. Trolling aside, Amphilophus robertsoni males get a beautiful blue when they mature. pretty slow growers though. Fairly tough, but at the same time, not terribly aggressive either.
Likewise, you could try an EBJD, but they are quite timid. Blue acara, Honduran red point, blue lobster [LOL can't stand when grown men see those crayfish and explain 'WOW! blue lobster? wait a minute......is this salt water!?'
Maybe you could try a nicuraguense, not totally blue but definitely some beautiful blues in the face and lateral line.