First of all this link you posted:
http://boston.fbi.gov/dojpressrel/pr...poss021009.htm
Was in reference to a felon possessing 1000 .50 cal shell casings. That has nothing to do with black powder or black powder guns. Shell casings are a component of modern, centerfire ammunition.
Second, yes, black powder is on the list of explosives, however, as stated in Federal regulations, a quantity of less than 50 lbs used for sporting purposes is specifically NOT considered possession of an explosive. Use in a black powder rifle or pistol is a sporting purpose.