I have 2 black skirts, Natasha and Rocky, with the same thing happening. The spots look like the human equivelant of a mole. Each of the fish have spots where the front fin meets their bodies. Rocky has been around for a few months and also has one on the back by his dorsal fin and one under his eye. (Henceforth the name Rocky- he looks he just got out of fight.) Natasha's fin is missing where the black "mole" has grown. I am not sure that the condition has anything to do with her fin being missing. She and Rocky use to get beat up by the other 5 tetras all the time. I removed Rocky first and aside from these weird growths now he looks great. Natasha just got moved and her fins are starting to heal so I think she will probably recover her missing fin as well.
Neither fish act sick at all. In fact, they try to school with my Red Hump Ciclid and play tag with each other all day. (My cichlid, Vaska, apparently finds them amusing because they are the only fish she chase away.) When I put them with the other 5 Tetras they both get beat up pretty bad. So water chemistry or physical abuse apparently has nothing to do with condition. I shudder to think, but could it be some sort of tumor? Has any one heard anything else?