It was probably going to be a black woman, granted. All of the reporting was that Duckworth was in the running. There was reporting on Whitmer, too. Klobuchar was out speficially because of her record on policing in Minnesota as a white DA. To be fair, if Harris were white, there's no way she would be on the ticket, too. But that doesn't necessarily mean an Asian (or in Whitmer / Warren's cases, white) woman who didn't have that baggage was automatically going to be excluded. Maybe in the end it was going to be a black woman, but the reporting was that up until the end it wasn't exclusively black women being considered.
Or put another way, or at least obvious to me as I already stated here two weeks ago, after Black Lives Matter, there is no way it was going to be other than a Black woman.