My Great Grandmother many times removed was an American slave, or at least that is what my mother told me; I have never researched this issue for myself. But I don't have any obvious characteristics that are commonly associated with black people such as dark skin, curly hair, broad nose or full lips. And every form I fill out, I put down caucasion. I can also claim Native American because according to my father my Grandmother, his mother, was part Sioux. Of course as my step-grandmother was full Lakota, I can technically claim that as well, or so I have been told.
My sister is part Asian, my step-father is Mexican and my son is part Afghani - we are just a multi-racial family . Although I think both my sister and I only really identify ourselfs as caucasions. It is how we were raised and we never thought anything else of it (except after 9/11 - I was a bit worried about my son, but I don't think anyone else recognizes that he is part Afghani because both myself and his very blonde, blue eyed father glow in the dark we are so pale!).
So, this all brings me down to my point - it doesn't really matter what ones ancestory is or the color of ones skin - it is the person who is now that counts.
I have also heard once that about 70% of people who identify themselfs as white Americans in the United States have some black ancestory.