He might also be a clone, maybe Madaras clone or something, but he isn't Zetsu. Remember when Zetsu said to Tobi, that he can't move at the freakin speed of light even he can fuse with the land. So if he is Zetsu, why should Zetsu be slower than Zetsu^^
White Zetsus are clones from Hashiramas DNA, so maybe black Zetsu is from Madaras DNA. And White/Black Zetsu was some sort of prototype of combining those 2. Kishi seams to play alot with those opposits, Black/White, Ying/Yang, Madara/Hashirama, Sasuke/Naruto...
Tobi could be Izuna or a Madara clone. But fore sure, he's got Hashiramas DNA (Izanagi). This might be the reason for that white "blood".