you have to be careful with "black clowns" or "black percula" they are often polymnus mislabeled. True Black ocellaris usually run between $40 and $70 so if it's significantly less it is probably not an ocellaris. Once you've seen a number of black ocellaris and black and white polymnus they're pretty easy to tell apart but the easiest way, if you aren't very familiar with them, is to look for yellow on the edges of the fins and tails. If there is any yellow it is polymnus and not ocellaris.