Black Lightning is a unique form of Lightning Release utilised by the Third Raikage, who passed it on to Darui. Not much is known about it but as the name suggests, the lightning produced by this ability's techniques is black in colour.
Both users have the stylised kanji for "lightning" (雷, kaminari) tattooed on their shoulders; Darui on his left and the Raikage on his right. This seems to be a distinctive trait, as Ginkaku and Kinkaku recognised Darui's possession of this ability through his tattoo.
It is similar to Amaterasu and Black Rain in that it is the only other black-coloured elemental technique.