i see this dark colored dust in just about every wareshouse i've ever been in that isn't very well ventilated. it gets on the racks and on top of just about everything if the place isn't kept clean on a regular basis.
it is a combination of forklift traffic and dock doors opening and closing letting in air borne dust in from the outside. Dust stirring up from inside trailers at the dock doors, etc etc. And yes even the tires can leave a residue on the floor even if the operator doesnt spin the tires. And of course in some cases if the lift truck is not tuned up properly it will emmit a carbon dust from unburnt fuel (in alot of cases you dont even see it but its there) if you have electric trucks then rule this factor out.

a good air ventilation system and regular house cleaning can help keep this down to a minimum ;o)