What is the material the soot is on, metal or brick or bricklike (refractory)? Whatever it is first vacuum the area with a shop vac with a brush attachment so not to scratch the carbon into the brick material or scratch the metal.

For metal get most off with soap and water. For the hard to remove carbon use Spray 9 cleaner. Try a small area to be sure it doesn't remove the paint, it shouldn't. If it does use a cleaner like Fantastic.

For brick or refractory material use tri sodium phosphate solution diluted in water. It won't be pristine but will look cleaner than it is.

Elbow grease is required also.