I think the official standard in Louisiana during slavery days was that as little as 1/8th African ancestry could make a person eligible for slavery. They were called Octoroons, and many were fair skinned with Caucasian facial features and were accepted as whites in the north and in some instances passed as whites in the south.
But in some southern states I think there was a "one-drop" standard which made anyone with "one drop" of African blood liable to be kept in slavery. I suppose most of those "one drop" individuals were the legacy of those randy slave owners who forced their "affections" on female slaves for the previous 2-1/2 centuries.
There was even a "quadroon ballroom" where the most beautiful quadroon women were romanced in New Orleans.