They do all sorts of things :-?
I have had orange fish develop black stripes down both sides of their bodies as they aged - looked very striking.
My disease book only mentions 2 reasons for fish turning black...
The whole body turns black - neural pathways which control fish colour are pinched or diseased.
The skin of the fish turns deep black in closely adjacent areas. These areas thicken and the scales stand out - the fish is affected with melanosarcoma, a malignant growth. Sacrifice the fish humanely.