There was a recent poll of over 800 Black voters and 50% of them viewed Kamala favorably and 18% unfavorably.Harris doesn't have the support of all black people, especially black men.
Another thing that could be a problem for picking Harris is that she isn't ADOS/FBA black.
Also, President Obama isn't "ADOS/FBA" at all and yet he won 93-95% of the Black vote.