The 2016 Democratic primary map. Blue is Clinton, Green is Sanders. 
Interactive version here. nytimes.com/elections/2016…
The Rust Belt places where Clinton beat Sanders were in urban areas with larger concentrations of black voters.
Exit polling, however, suggests that age was a huge factor in terms of the black primary vote in 2016 (as it is in 2020). Younger black voters were far more inclined to support Sanders than their elders.
