To my knowledge each black has different genetic backgrounds. I can't pin point the SB's background yet (I get different unlikely infos), since I've only started to work with them and they are . . . very weak, or the ones I bred were (fry won't eat). All I know is that they have a marble background.
Melanos are supposed to be "true" blacks. But IMO their color isn't as intense as other blacks. And since the females are mostly, if not all, sterile, melano males must be crossed to other colors (usually blue) thus they are often bi-color.