Yes, indeed the -535s can get a bit sooty, especially during start-up. In fact, I was watching the PHL
webcam from http://www.jfktower.com/liveatc.htm
a few weeks ago when I noticed a plume of blue smoke just barely within the view of the camera. When a UA
757 taxied by a few seconds later, it had become clear that it was the one that was emitting the smoke. In your case, it was probably just residual fuel from start-up or the previous shutdown being ignited in the tailpipe during takeoff. Rather common on 757s.
B-52s don't take off. They scare the ground away.