I need help with my betta fish!! please help!?

Ok so a few months ago i got my betta fish which i named Poseidon. When i would feed him he would gobble up all his food and want more. Now when i feed him he wont even go near it. He is in a big tank and his water it pretty much all the time 76- 77 degrees. I have gotten him new food cause i thought that maybe he got sick of it. He has never liked flakes even when i first got him. Then i went out and got him freeze dryed blood worms and he wont eat them too.. He used to be a vibrant blue and now his color is very very very dull almost black. His water is clean. I am very confused please help me. ! :(