Could be either. Black "dirt" is what the fleas leave behind - if you wet it, it turns red cause its digested blood from the animal. Flea eggs are white.
Do you notice this "dirt" on the rest of his body? If not, then he's probably stuck his face somewhere dirty or dusty. I'd go over the entire cat before you panic about fleas. If its just dirt and not fleas, then give him a good bath and try to find out where he's getting dirty.
If its flease, then you will have to start treating him for him with Frontline or something similar. Do NOT put a flea collar on him.