It's rather clear by now that Madara is just a bitter old eyeball living through a body of Zetsu goo. And that, in a way, Madara, after his fight with shodai, managed to survive through the creation of Zetsu, which gave him a body of sort, for his angry remaining eyeball. But what if Madara himself, by mixing Shodai's cell with his own, was the source of black Zetsu
White Zetsu = Shodai's cells.
Black Zetsu = Madara's own cells.
Would actually make a lot of sense. Black (Darkness/Uchiha/Madara) and White(Light/Senju/Shodai) incarnated. It was, after all, Madara's goal to mix the two