Black and White Zetsu may have been created as a body or shell for Izuna to be resurrected. In the Anime episode 136 where Itachi explains Madara and Izuna's past to Sasuke...after slaying fellow Uchiha to obtain MS, there is a scene where they are both looking up at the moon...which happens to be a half eclipse(part black and part white). Izuna was just as strong as Madara, I refuse to believe that his character was killed off that easy. Maybe Zetsu was Madara's first attempt and then Obito's accident provided him with a second opportunity to combine Senju and Uchiha DNA for the perfect vessel for Izuna. This may sound dumb but it's possible that Izuna now refers to himself as Obito because he no longer inhabits his original body...just a thought. Madara has way too much respect for Tobi which doesn't make sense if Tobi is just Obito. I would have whooped Obito's ass for not resurrecting me as planned if he was just my student.