Warren cannot get high turnout by black voters—a black VP is _not_ going to get it done.
Blacks don’t trust her—and the optics of a black in an “inferior” position will just damage that trust even further.
It will be (correctly) seen as elite white pandering.
2016 may have been different in that by picking Pence Trump helped himself with conservatives and evangelicals who were wary because of his earlier life. And picking LBJ may have made the difference in 1960--LBJ may not have made an effort to steal Texas for the ticket if he was not on it.