No one is claiming that people don't believe in race. There's just no supporting evidence genetically, making race a purely phenotypical analysis, which is a very poor indicator itself of heritage. One of my dearest friends in high school was from Dominican Republic. She spoke with the dialect (speech patterns, word choice, intonation, pronunciation) of "white" people, but to look at her, she appears mixed. She is not mixed at all (by the usual race-driven definition of mixed), that's just the expression of her genes. Unfortunately, by the white-black divide so prevalent in American racism, she would be labeled black, and no further consideration given. She not her ancestors (by any reasonable measure of generations) are from Africa. Yet, she's "black."
And for the record, "black" is not a race.
But since you trust common folk to understand something so innate to this world and so simple that it confounds science, ask them what race someone is. "White," "black" (African American if they're PC), "Mexican," and "Asian" are your likely answers. You may get "brown" for Mexican if they're particularly fond of color designations for these mythical races. But, krellin,
Colors are not races, and
People are not colors.