It's mestizo, and that is a racial designation, not a nationality. Even in the 1920s, when Mexico was removing race from its census forms, the United States Census Bureau was estimating three-quarters of African Americans could claim at least some white blood. Few are willing, in the face of what appears to be omnipotent, omnipresent white racism, to split hairs or chromosomes over the one-drop rule. It is the easiest advice for parents to give. And the most difficult for teens, still forming a sense of their own individual identity while also yearning to be part of a crowd, to follow.
Multiracial children bear a special burden.
They hear black people instructing them to identify strictly as black because that is the way society will view them. At bottom, it is the society of those very same black people. Black Americans who have internalized white supremacist attitudes and values become agents of those attitudes and values, enforcing them in others and passing them on to new generations more effectively than the Ku Klux Klan ever could. Biracials point out that they are running away from nothing. I believe them.