Betta Fish: Male Samurai Betta losing black color??

Hi! This is our first betta. He’s my son’s fish and he’s very well taken care of. Water tests all come back good, he has a 10 gallon tank, and he’s fed high quality food. Temp is great/etc. We got him in October and he was very dark black in. Recently we’ve noticed his black is almost fading!? He’s otherwise healthy (from what I can tell). He has a small nick on his tail fin (not sure what from as we only have betta safe decor and plants) but it seemed the fading color started before the fin nick. The first pic is when we got him, second from today. Thoughts?!