America has already elected a Black candidate for president, and Harris is half Asian Indian and even lighter skin toned that obama, so her being ‘black’ is not an issue for the general election.
Also, you are forgetting about identity politics and the sting that a lot of lefty women feel about the ‘First Woman to Get Elected President’ getting Trumped. If Biden somehow doesn’t Biden his way out if first place but doesn’t have enough delegates for the nomination, he’ll have to recruit Harris to run with him in order for enough women to support him in the general election. Beto’s only shot at being the Veep nom is if eother Harris or Warren are on the top of the ticket.