Social (dis)order over the generations.
During slavery the masters "took liberties" with black women. The resultant offspring was not acknowledged officially by the white race as word/heresay/inuendo got out. Blacks ultimately ended up taking the offspring in and raising the offspring. Couple this with the stripping of any social standards that were in place in africa. Plus two, since whites controlled the country and made the laws and are basically europeans with a keen sense of racial identity, they passed laws that described what people are especially if they have black blood in em. Now it is so engrained in our society to take in the "outcasts" that it is a natural to consider anyone mixed with black in this country "BLACK". It's not fair, but I think it is simply a knee-jerk reaction by the black race because that's the way we have been indoctrinated to see things.
Logically they are mixed, not black or white or whatever, but logic has no clought in these type issues.