Richardson is hispanic, so he is out. A black and a hispanic will not work.
Why not?
For purely practical purposes. Voters will be deciding whether to elect the first black President, if Obama is the nominee. Obama does not need the additional challenge to voters to elect an all minority ticket.
For all practical political purposes, Obama's running mate has to be very obviously white, preferably a white male.
An all minority ticket? In that case, Obama could kiss the Presidency goodbye. Guaranteed. That would not fly, especially with white voters, in particular white male voters.
Guaranteed, if Obama is the nominee, his running mate will be a white male.
I do not buy in the least what some say that Obama offered Richardson the Vice Presidential nomination in return for his endorsement. That is ridiculous, and would be a tremendous blow to Obama and to his message of doing things differently.