No, he never actually said his VP had to be a black woman.
He said that we didn't want a white male and that he wanted "someone who was of color and/or a different gender". In another speech he stated "I commit that I will, in fact, appoint a, pick a woman to be vice president,".
So the VP had to be female and a person of color (person of color is someone who is not white. So could have been Black, Latino, Asian, etc).