Sooty Exhaust ?
 

Ever since I had my "clogged" cats changed out to a bonez high flo cat, My rear bumper is starting to look charcoal colored. Its all black and sooty and is kinda grainy. Any Idea what is causing this ? Maybe the car is running to rich or something ? I dunno its really pissing me off having to wipe the bumpers off every other day cause it looks dirty

Yes running rich is the main reason for this, mine was doing the samething