The US racial classification system is so bogus that it is amusing that people take the stuff so seriously!
I dislike the term 'sub Saharan' so vehemently that I create my own terminology so it behoves one to reframe any obviously absurd geographic designation meant to denigrate, in the first palce. Better to look at the origin of something and come to that logical conclusion.
Jast as people have a concept of Northern Europe, Southern Europe, Eastern Europe and Western Europe within that homogenous frame of references(s) Africa n totality being far more heterogenous in its Northern, Southern, Eastern and Western points. No difference. I see no similar sub X designation in Europe so why doesit exist in Africa?
Objectively, definitions are in order so US classified blacks cannot have white children. That being said, degree of mixed or unmixed ancestry (I would imagine recessive and dominant genes of 2 or more ethnicities ) would practically indicate the offspring. The observer present will also impact the definition of 'black' and 'white'. Self identification within the US racial classification would also be a factor.
Lolo Jones, based on her personehood, would be assessed a certain way based on her social milieu, self identification, how she is coiffured, speech pattern identification, etc, The same would apply to Charles Rangel and others.