That was only true in some states during Jim Crow. Others had different definitions of "white". In fact it was possible for some "black" people to become "white" just by crossing a state boundary.
Just checked that there are 7 millions Jews in the US. That's a lot more than 1%. About 3 millions are Slavic people. Many millions are european-type Arabic people. They all look like white Americans and if they grew up in the US, they won't even have an accent. The number of "true north-european descendats" must be a lot lower, maybe 60% of white Americans. And this number is going downhill rapidly because Americans don't hesitate to mix with other races and nationalities. My point is that albedo of skin is a really poor predictor of ancestry, about as good as a coin toss.