One thing is almost certain about Black.

I'm 99% sure Black is a God. The way he belittles mortals in similar fashion as Zamasu, and the fact he uses God Ki, I doubt he trained like Goku and Vegeta to obtain it, I think it came natural to him like Zamasu. Based on what we know so far, there's a 50-50 chance of who he might be. a) He is the God of Destruction like my original theory, we know Zamasu is now immortal, but what if he cease to exist if the God of Destruction (Black) dies? b) I heard another theory here that sounds quite reasonable, what if Black is Zamasu from another timeline? We already know there's two Zamasu, one in Goku's present and one in Trunks future. Zamasu abusing the Time Ring, may have created an unlimited amount of timelines, if so, it's not far fetched to believe that Black is a third Zamasu.