I got 3 chromis when I started and 2 of them developed a single dark black spot and then died. One lived for a good week with the spot, but it eventually became this huge black growth that was as wide across as the poor little chromis. I never figured out what they were. I don't know if its the same thing you are seeing because on each of mine it was just a single black dot that grew into a large spot before they died. It looked fungal to me at the time but if it were to happen again I think I'd pull the fish and treat it with a maracyn/maracyn 2 antibiotic combo to see if that made any difference. Not suggesting you should do this because I have no idea what your fish have, but if what happened to my chromis were to happen again, this is the route I would try.