Ian's comment sent me to the chemical resources to find out about this before I commented.
"Black powder contains free carbon in the charcoal and is coated with graphite, another form of free carbon. This makes it conductive and able to carry a small current with out heating to the point of ignition. One of the points of the graphite is to reduce the static discharge hazard."
However, large quantities in one container carry more risk than many small containers. When I was young, a long time ago, DuPont sold black in metal one pounders, but anything over four was in a wooden keg. But I don't know why, it just may have been in there for 50 years or more.