Digimon from "black series" are (in most cases) virus and/or corrupted counterparts of other Digimons. They born like all other Digimon*. A good Digimon may also skip into its "black" form if gets corrupted. * OBS: The BlackWarGreymon from Digimon Adventure 2 is an exception because he is an artifficial being created fusing 100 control spires.