I have read about this happening. Apparently sometimes if a fish hits itself a nerve will become damaged and that area will turn black. It's a neurological injury. Don't ask me how it works, but I've heard about this happening in the past. It does NOT affect the fish's health otherwise. I have seen it in a few fish including firemouths, convicts, guppies, loaches and so on.
Your guy has gorgeous markings and orange!