Hmmm...
I don't think hip hop is essentially black... but it's still a dominantly black artform in the ends conceived in the overall effect of the sum of it's parts... The combination of the imagery, feel, etc, constitute an expression that is still reflective of, and rooted in black youth culture.
There's nothing so inherently "black" about rap music that it can't be deteritorrialized by another culture and re-imagined to suit that's culture's image and sensibities. (just look at the different costumes that techno and rock have worn over the years...) However, the reason I'd still call hip hop a "black style," (as politically-incorrect as that sounds as I type it), is in the difference between rap's foray into the white mainstream, and rock 'n roll's. Despite hip hop's popularity and absorption into mainstream America, most popular hip hop is still primarily made by black artists, and is still the most popular style amongst black American youth (and it's feel, content, slang, beats, etc. still reflect current black American trends and taste). Not just that... surprisingly, unless you count rap-metal, the form hasn't yet been turned into something more "white" to suit white middle-America, despite it's popularity (although there is the theory that the emphasis on bling and gangsterism is perpetuating a cheap, bad stereotype that suburban kids buy into).
Even white hip hop artists, instead of changing their music to suit stereotypical "white" sensibilities (unless you count half-assed, ironic hipster hip hop novelty acts), usually seem to strive to be accepted in the "real" scene, and make music that plays to black-influenced sensibilities, as opposed to splitting off into a more "white" form of hip hop completely distinct from it's origins and original feel, as was the case with young white America co-opting rock 'n roll.
To me, a style of music can be credited to an ethnicity or culture, when it could only have been conceived by that particular culture in it's particular place and time. The style might eventually become a universal meme, and some of it's offshoots may stray so far from their original source that they evolve into a completely new form, only reflective of it's current milieu (which is IMO, why rock is a "white" style, while rock 'n roll is a black style), but a style's original form belongs to the culture that birthed it. (I'm not saying only the originators can use it, but it would be kind of insulting to say for instance that jazz or blues wasn't a originally a product of black America).