• Belt dust from a belt that has been rubbing on a belt guide.

• Exhaust soot from an engine that is running to rich.

• A drive shaft contacting an intake boot.

• Grass screen rubbing on the blower housing.

All of these are possibilities but this final one could be the culprit. 

A turf disease called Slime Mold could be the cause of the soot or dust. Slime mold is a dark gray to black crust like material that forms on the leaves and stems of turf grass. This soot-type material rubs off easily on shoes, clothing a, and can be picked up and become airborne by wind or mower blades. Once this happens it will settle on anything in the area that it comes in contact with. (The lawnmower deck) 

Slime mold Fungi feeds on decaying organic matter splashed upon the leaves and stems of grass. Slime mold does not feed on the grass or cause damage other than giving the lawn a discolored look. Slime mold can be removed from the grass by applying water under pressure or by mowing. No chemicals are necessary to rid the lawn of this fungus. 

So the next time a customer asks you to look at his mower because there is a black dust all over the deck, keep in mind that it may be caused by Mother Nature.