Fuel mixture is controlled by the PCM and there is no user adjustment provided to change it. There are several things that may cause it to feel it needs to run rich. Have you had the codes read recently? That would tell you if any pertinent sensors (intake temp, oxygen, engine temp) need replacement or if any other problems exist. Has the car been getting worse mileage? Also, if the dealer plugs into the car, he can see mixture over time as well as real-time and confirm.
FYI, my tailpipes on my Saturns are always black. I'd tend to think that's normal since I read my codes frequently and know everything is operating as expected.
Lane [ l a n e @ p a i r . c o m ]