Okay at the valley of the end Sasuke's chidori turned black because of the curse mark, originally the colour was thought to just symbolize that he was using evil chakra but what else was in the curse seal? The curse mark is derived from Jugo's nature absorption, and that Oro puts his senjutsu in the curse mark. However Darui and the third Raikage aren't so that could mean that black lightning is lightning mixed with senjutsu chakra therefore my point is.... (http://narutobase.net/forums/showthread.php?t=233999 - thread where I found this)

I WAS A SAGE BEFORE ALL OF YOU LALALALAAL SAGE OF KUMO REPRESENT!