This just may be the result of doing mostely short trip driving in the "econo" mode. My '95 does the same thing. Shifting to "sport" mode might get rid of some of that soot but your mileage would surely suffer. These days though soot's alot cheaper than gas. Could also be engine temp sensor slow response or maybe low output of air pump??? Don't know how to test air pump output. Does your engine temp gauge get up to the normal temp quickly? Ten+ years is a stretch for most thermostats. If your's is broken and open all the time the engine could not be warming up quickly and running rich longer?