Helene
Perhaps its just accumulations of natural pigmentation (melanin). Appearances of what seems un natural colouration can be common and in some cases aesthetically pleasing eg .Black Angel fish. The appearance in a number of related fishes could be due to a genetic marker (same with the malputta – and perhaps its a coincidence) but what triggers this I dont know ..I guess there could be a lot causes and one of those could be environmental… But at the end of the day if the fish are healthy I perhaps would not worry to much about it.