It's not a black thing per se.
It started in prison where they were no belts and the clothes offerred were loose. The prisoner's got used to this and so kept up the trend when they were released.
My son (15) tells me it looks "well gay" that they look like this. He's a good lad
Its supposed to be "gangsta". Street. Its how all the rappers blurt their lyrics about bootys and guns and being well hard etc. Ali G took the piss out of it brilliantly.
Makes them sound a bit retarded, but I guess that just comes with the territory.