In this area it is not so much Blacks as White kids.
I've noticed that young white suburban kids tend to be the main culprits of the loud, thumping car stereos playing rap music.
My guess is that many black kids probably live in an area where a $5000 car stereo will be stolen in the blink of an eye and thus don't want to spend that much money on one.
My younger brother was one of these kids. Used to drive me crazy! He spent thousands on his stereo till he had a car with literally no trunk or back seat space. All speakers and amps. Eventually he grew out of it.