Exhaust system soot?

I recently changed the exhaust system - cat back to a new Bosal unit. Old muffler had a few holes. I noticed the interior of the old exhaust pipe has a complete coating of black soot. My truck is an 89 22RE 5 speed with 90K original miles on it. I have had it 5 years and have only put about 6K on it with mostly real short trips. Is that he cause of the soot? I get pretty good gas mileage when I drive more than short trips.

The truck is pretty slow to warm up - going by the temp gauge and never runs mor than a third of the way to the red. It has absolutelu no drivability problems. Is the soot normal.

Perfectly normal, the burning of fuel produces carbon. Your new cat back will get black eventually.