It's a similar genetic hiccup to albino or some other color morph only in the opposite direction. Off the top of my head, I'd call it a "melanistic RTC", the antipode of the albino RTC, is all.
You can call it black panda or pink panther or tender-baby-salad-green-colored elephant, if it helps you sell it better. There will be those who will happily gobble it up and thank you for it.
Remember, genetic perversions do not occur one by one but in bunches and if a fish differs from the norm appearance-wise, it also differs from the norm in its internal built, organs, nervous system, etc. and can be guaranteed to not reach the growth rate, size, health, vigor, vitality, immune system strength, and longevity of the norm, and the said deviation can vary wildly but it will for certain fall short of the norm given the same adequate care.
So beware that you may pay $500-$1000 for your color morph or short-body or somehow else perverted RTC, or $60,000 for your albino RTC only for it to kick the bucket for no obvious reason 1, 2, 3, or 5 years later. Sometimes it happens in months from what I hear. Sometimes in years.