In the US, anyone with any African ancestry is considered to be a member of the minority group generally designated as African-American or Black, formerly Negro, Colored, etc. This undoubtedly is because our laws, back during the time of slavery and also during the more recent "Jim Crow" era, defined anyone with one-sixteenth African "blood" (i.e., DNA or ancestry) as "Negro." That's a person with fifteen white great-great-grandparents and one black one.

For this reason, I believe, all Americans ~ black or white, young or old, prejudiced or not ~ seem to "recognize" any person with the slightest visible trait signaling African ancestry to be Black.

I'm sure that things are much different in, say, South Africa, where there were also racial laws, but a different set of laws that differentiated among three groups: white, black, and "colored," i.e., of mixed African and European heritage. South Africans of all races, therefore, grew up with a completely different view of race, and recognizing a different set of visual clues for a given individual's racial identity.

A South African would view most African-Americans as "colored" rather than "black," because purely African ancestry is actually quite rare on this side of the ocean.

This is a sore subject to many folks, because much of the Caucasian element in the African-American gene pool is the result of slaveowner rape; nevertheless, almost all American "blacks" are actually, to some extent, of mixed heritage. (There is plenty of Native-American "Indian" ancestry among African-Americans, too, along with the European element.)

Nowadays, whether a person considers himself "biracial" or "black" depends mostly upon how recent his Caucasian forebearer(s) entered the gene pool. People like Barack Obama (white mother, black African father) and Derek Jeter (white mother, mostly-black African-American father) are seen by many (if not by themselves) as "biracial," not "black."

On the other hand, the large "Creole" population of New Orleans consists largely of people who are partly black and mostly white, and all of whose parents, grandparents, and great-grands were also of mixed heritage and therefore ~ according to the laws and customs of their time and place ~ seen by themselves and by others as "black," even though many are quite aware of numerous white ancestors, and some even know some white cousins, even entire white "branches" of their families.

Yes, of course, we all know (or should know) that "race" does not have any real scientific basis, and that in truth there is only one race, the human race. But on some level racial differences continue to exist, even if we can come to an understanding that the differences are primarily cultural and not genetic. Taking this relatively "enlightened" viewpoint into consideration, I can see how a person growing up in a household with one white and one black parent would understand him/herself as "biracial." By the same token, another youngster who is also half-black and half-white, but in this case because all eight of his/her great-grandparents were half-black and half-white, that kid would have grown up in a solidly African-American community and would identify simply as Black, not as "biracial."