85%? Lolno.
You seem to forget the fact that there are African-Americans whose ancestors were slaves forcefully brought here, not as immigrants, and also Mexicans living in Texas, New Mexico, Arizona, Colorado, Utah, and California when those areas were part of Mexico and not the U.S.
Also, the true number is likely closer to 97% due to interracial marriage as you said, but I wanted to go for a conservative estimate and err on the side of caution.