With the native, non-hybrid, black crappie, the black stripe is just a genetic trait that some inherit, like blue eyes or whatever in humans. I catch one every now and then. From what I understand, one of the hybrid-strains that are bred for ponds are black striped. I think they deliberately used black striped crappie in the cross to produce hybrids with black noses to more easily identify them. I could be wrong about that though.