Sure, the progressives and BLM folks will like it and there won’t be criticism from those quarters, but it’s not like you can lose those voters to Trump.
I think not picking a woman of color would actually cost him votes. He needs black voters to come out. Not doing so cost them 2016.
Plus, he was under so much pressure to pick a woman of colour. Klobuchar dropped out for that reason putting even more pressure on him, intentionally or not.