I think that Niles is right that there really is no such thing as a "black" or "white" race, that these identities have to do with attitudes and identities related to crucial historical experiences, in particular that of slavery and its aftermath.
Let me add some further observations on this complicated business of "racial" and "ethnic" identity, which depends heavily on one's own self-identification and personal experience. Thus, there are many "blacks" who have not only some African ancestry but also significant inputs of European ancestry and in some cases Native American Indian ancestry as well, Niles providing the example of the apparently "white" brother in his high school. One difference between "blacks" and "whites" in the US is that the historical experience of slavery meant that the separate tribal identities from Africa were largely submerged and even destroyed, with the exception of some specific remnants among the Gullah, thus in some sense creating a new synthetic group whose self-identity has gone through a tortuous evolution as evidenced by the many names that this group has used for itself over the years ("colored", "Negro", "Afro-American", "Black", "African American", "People of Color" (a broader category admittedly), etc.), a difficulty reflecting continuing oppression and discrimination at the hands of the dominant "whites".
This suppression of tribal identity has not happened so much among "whites," with many still remembering and identifying with their ancestry from one or several European nations. The emergence of the concept of "whiteness" can be seen as arising out of the racial laws regarding slavery that arose in the late 1600s and early 1700s in Virginia in particular, the privileges of avoiding slavery that would attend to anyone being identified as "white." This identity, associated with not being "black" and thus subject to slavery became convenient for the later European immigrants for whom it would gradually become a badge of identification.
On a less serious note, and in response to Charles Brown and some others, I suppose we could have an "oppressed peoples contest" on this list, in which everybody recites their descent from some group or other that has suffered from oppression or discrimination or slaughter or whatever. I'll start by noting that I have gypsy ancestry, wow, hot stuff! Furthermore, just to complicate things and to add an element of mystery, :-), I note that although my last name (Rosser) is of Welsh origin, there are many people in the US bearing that last name whose paternal ancestral line derives either from Africans or from Jews. I won't say which one mine derives from other than to note that the majority of may ancestry does not derive from that particular source anyway. Hah! Yes, all this is pretty complicated. Barkley Rosser
-- Rosser Jr, John Barkley rosserjb at jmu.edu