I too have a couple of clowns that have these same black spots. For the past few days I have been researching and have come to the conclusion the spots are likely the results of a coral stinging the fish. Do you have any Euphyllia (frogspawn, hammer, torch, etc)? I have seen many posts about other people's clowns with black spots when they are stung by euphyllia. The spots are just excess pigmentation that form when they are stung (sort of like a mole I guess)... You might want to read up on black ich too though. However, what I have read said that black ich looks like many tiny dots, about the size of a pin head. Hope this helps.