Maybe he has a touch of HLLE? I can't really tell from the photo. Do you run carbon?
I would try feeding live worms and vitamin-soaked frozen foods/nori to help boost his immune system. Blue tangs are very susceptible to HLLE, and it's often something missing from their diet.
I don't think the fish is sick there was 3 hippo tangs that came in there were about 50 or so and they had strange colorations the other 2 hippo tangs had mostly black
There can be a lot of individual variance in the amount of black on a hippo tang and it can also vary as they mature. They are usually somewhere between the examples you have shown.