when most black or white Americans whose family has been here more than a couple generations are mixed black and white
I would further specify that their families should be recognizably mixed black and white, as most individual African Americans and many whites have had mixed ancestry for hundreds of years. On that note, individual estimates of ancestry will soon be available to anyone who wants them, which could also change things in some unpredictable fashion.
We’d probably also have to stipulate extended families, as I doubt it is likely for “most” say nuclear + 1 families that include a black or white person to also include the other, simply because blacks are a small and declining minority and whites, while a majority, are rapidly declining as a percentage of total population (all referring to the US).
In any case, here’s to more race mixing and the destruction of race as anything other than a historical concept.