I think CCM is a good choice. Like you said, Biden doesn’t need any help with black people and Harris is actually less popular with the black community than he is. What he needs is Hispanic support, and CCM fits that while also being fairly young, non-controversial (by politician standards) and from a lean-D state.
Was 2016 wiped from your memory? Hillary won more of the Black vote than Biden has up to this point and turnout still cratered. Hispanics are not even reliable voters, and this woman has no receipts being on the frontline on issues that disproportionately affect them. IDK why y'all think she would have lead this huge surge to the polls for a group that is way more unreliable than even the most unreliable bloc of Black votes.
Biden needs extraordinary turnout from Black voters. And all the potential VPs are "less popular" with Black voters than Biden because they are less known. Harris' favorability is within the margin of error with Warren and Abrams.