The powdery black stuff that's sometimes created when fuel burns is called soot. A chimney sweep's job is to clean all the soot and ash from inside a chimney.
Soot is mostly made of carbon, and it forms when matter burns incompletely. Engines, burning coal, and house fires are all sources of soot, and soot is a major contributor to air pollution around the world. It's dangerous for people to breathe too much soot into their lungs. Soot comes from a Germanic root that literally means "what settles."