http://www.youtube.com/watch?v=Xtoanes_L_g
bell hooks touches on your point about hip hop.
i think it has a lot to do with the spending power of white middle class and working class males in america. males who are likely to have some contact with the black racial other, but also experience white privilege and status. in their adolescence, their fantasies of the "authenticity" of this black other whose life will seem to offer a form of "realness" (due to its obviously different characteristics and lower socio economic environment). this lifestyle would seem to offer a break and alternative to the stultifying white world of conservatism and the inertia of greater security. therefore, pretty quickly both blacks and music marketers work out that they can present a skewered commodity version of this lifestyle and make money off this. now its simply neater and easier to fence off hip hop as a "black" thing, thereby cutting out all the messiness of mexican/hispanic racial others that intrude on the fantasy. because if anything the recognition that this "specialness" was not down to some sort of aesthetic (and therefore imititatable) characteristics but in fact down to purely socio economic conditions would ruin the hip hop fantasies' notion of excessive capitalist greed and success.