You'd be surprised. It depends on how far back you have to go to get to his nearest pure-African ancestor.
My family is all blue-eyed blonds and brunettes, but imagine our surprise when genealogical study showed the only man on a Rhode Island census of 1774 with the same name as my great-great-great-great-grandfather, Valentine Brown, was listed as 'colored'.
We knew he was born in Rhode Island in 1755 and served as a Revolutionary War soldier in an unknown Rhode Island regiment-- but he turned up in Saratoga, New York, after the war, and we know this is the site at which one of the Negro regiments from Rhode Island was demobilized after the war.
We had always wondered about this man, as centuries of family tradition held that he was part of the well-known Brown family of Rhode Island, and yet we could find out nothing about his parents. This is the same family that started Brown University, and its members for many generations have been well-documented genealogically. They were also major slave traders, running the triangle route-- rum was traded to Africans for slaves, the slaves were traded for molasses in the Caribbean, and the molasses was used to make more rum in Providence.
In Saratoga, he married a young widow with children, and ever since then, their descendants have passed for white and never considered themselves anything else. I have pictures of this man's grandson and great-grandson (my great-grandfather and great-great-grandfather) from the late 1800s. They show men of Mediterranean appearance, with dark curly hair and dark eyes, but otherwise clearly Caucasian.
Our conjecture is that old Valentine was one-half or one-quarter African blood, probably the illegitimate son of a scion of the Brown family who forced himself on a slave girl. In Saratoga, which was on the fringes of civilization in those days, few would have noticed or cared if he took up with a white widow with hungry children and few options. Such situations were more common than we think, and in the 1700s in the North, there was not the same social stigma and prejudice as in the South.
It's hard to prove such things conclusively at a distance of two hundred and fifty years, but it is certainly plausible and likely. A hundred or even fifty years ago, it would have caused my ancestors considerable consternation if this were widely known, but in today's world it is just a mild curiosity, another thread in the great American tapestry.
-ccm
Does this make you eligible for minority entitlements??
Just kidding. You're right, after generations of being watered down, a certain racial look can easily begin to fade.