The US racial or ethnic classification "black" refers to people with all possible kinds of skin pigmentation, from the darkest through to the very lightest skin colors, including albinos, if they are believed by others to have West African ancestry (in any discernible percentage), or to exhibit cultural traits associated with being "African American". As a result, in the United States the term "black people" is not an indicator of skin color or ethnic origin but is instead a socially based racial classification related to being African American, with a family history associated with institutionalized slavery. Relatively dark-skinned people can be classified as white if they fulfill other social criteria of "whiteness", and relatively light-skinned people can be classified as black if they fulfill the social criteria for "blackness" in a particular setting.
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): How are people defined as "black" or "white"?
they fulfill the social criteria