The appearance of soot in the gas stream shows incomplete combustion. This means insufficient combustion time, or not enough oxygen.
Now, depending on the fuel source this can mean that it is essentially pure carbon which can be used as a pigment. Anytime you see black plastic, it's about 1/2 to 2% carbon black. If the source is oil or coal there is likely be admixture of a real witches brew of partially oxidized hydrocarbons. I wouldn't want it on anything I grow in quantity.
Fortunately, in minor quantities most everything but the pure carbon can be eaten by some bacteria. (Generally if there is energy to be gotten out of a chemical reaction, there is a bacteria that will take advantage of it.)
A better solution is to better regulate the fuel/air supply at the source. Some care has to be done with this, as if the fires run too hot you don't get soot, but you do get NOx (Nitrous oxides) which produce smog and acid rain.
Also: Coal as a fuel produces fly ash. Very very fine ash. It comes out as grey dust, but in thick enough quantities can appear black. This is the stuff that isn't coal in the coal. Mostly silicon dioxide, calcium oxide, but traces of arsenic, lead, mercury, depending on the source of the coal.
More info here: https://en.wikipedia.org/wiki/Fly_ash