just curious, my friends trans am has this in a small area above his exhaust pipe.

 

I remember cleaning a 4th gen T/A (i'm a car detailer) at my dad's shop and it had some of this black film going all the way up the back and even some on the underside of the spoiler. The exhaust pipes were cut a lil short and didn't quite clear the bumper, so I guess over time it created the black film.