It's not only applicable to BP's, and the only reason BP's get it is because of their mixed breeding. It's actually a trait of Midas and Red Devil cichlids, which make up a very big portion of the BP's bloodlines. This is why I was asking. Black comes and goes on these fish all throughout their growth stage. They are known for going through numerous colour changes as juveniles. The black is usually just an indicator that something has changed - not necessarily bad, not necessarily good. It could mean the water quality has changed to a "bad" level, or it could just be that the temp changed, food changed, decor changed, tankmates changed, so on and so forth. It's really not something to worry about as long as you keep track of/maintain proper water params.