Dark patches sound like a stress reaction, like night camo coloration. A lot of tangs will get blotchy colors when they are stressed. Have you witnessed any aggression? I keep a pyroferus, one of the three species sold as mimic tangs and the one usually known as the chocolate tang. When I got him at around 2" he was solid yellow juvie, a Herald's angel mimic, and as he got larger he darkened around the head and especially the anal and dorsal fins and developed the dark and orange markings around the gills the adult is known for. Mine has darkened quite gradually over a period of years and continues to do so, your description does not sound like a normal color change, expecially since it came on so fast. How big is your fish and the tank he is in, any tankmates?
