well at this point with the kishi showing us so much about dark and light chakra manipulated jutsu in this arc alone and actually pointing it out to the readers its actually yin or yang based(for example kishi specially said genjutsu was yin based when the muikage was fighting even though we have seen genjutsu since the start of the series) is telling me that black lighning is a yin(dark) based form of lighning transformed chakra.
This is just like the black flames as the sharingan itself is yin based, this hasn't been explicitly said by kishi but its obvious at this point.
Even edo tensei will be a yin/yang based jutsu, tobirama talked alittle about this recently.
CHeck out my yin/yand chakra manipulation thread I did ages ago, everything I said months ago came true word for word.
Most likely it is a hidden technique... sort of like the Shadow Possession, Expansion Jutsu, and Mind Transfer; however, it requires Raiton Release....
