Blackness and whiteness are useful concepts when seperating artists and music genres into categories however that's as far as it goes. You cannot seperate blackness and whiteness simply as its based on a lot more and involves looking deeper into class and economic environment.Eminem is an artist who's music could be described as sounding like and fitting into the genre of black music, he is white yet the background and upbringing he has had has effected the type of music he chooses to produce.
Pop music has always depended upon the interaction between white and black traditions - David Hatch & Stephen Milward
To seperate music is to stereotype music and it would mean saying that typical genre's like rap or rnb could only be produced by black people, and that classical music or indie music is only produced by white people, these stereotypes are wrong. White artists have covered black music, like Elvis and The Beatles especially when Rock and Roll came about. Alot of the time Black artists produced rock songs just for white artists to cover it and then have their version released on the radio instead.
In my opinion there is a fine and blurry line between seperating blackness and whiteness in studying popular music, i don't think it is useful because everyone is going to have thier own idea's on what it means to them.