It all makes sense now. After meditating for years, I have finally come upon enlightenment. What you are about to read will give you in depth description of how Black Zetsu came about.

So recent chapters, we've come to know white zetsu and his personality. His interest in poo has struck me. The thought kept haunting me in my sleep and during the day. It wasn't until today where i finally come to understand him. You see, Black Zetsu was made from Obito's poo. "wow really, i read this only to say **** you"
No just hear me out.

How Black Zetsu came about?
Obito is ofcourse in a cave with Madara who is currently hibernating. Where do you think Obito goes to take a shit? That's right, inside the cave where the Zetsu can easily obtain it. White Zetsu took samples of Obito's poo and observed it and realized he could give it life form. He injected the poo with some Hashirama love and thus the birth of Black Zetsu. Ofcourse poo is of dark color unless Obito had some spicy ass shit which will then make it red. The darkness of the poo is what made the Zetsu come out to be black.

If you want to argue, please provide facts as to why I am wrong otherwise i will not respond because I am far superior and more knowledgeable than you could ever hope to be.

Thank you, you may now blow me!