Rules change daily. Less than 50lbs and have a easily verified purpose...
(1) the black powder is commercially manufactured;
(2) the quantity of black powder does not exceed 50 pounds;
(3) the black powder is intended to be used in antique firearms or antique devices solely for sporting, recreational, or cultural purposes.
Any other purpose requires an ATF permit. Its is an explosive powder after all.
Brand new black powder rifles/pistols are considered 'antique'?????? Oh wait, forgot about where its coming from.
Federal rules are one thing. You also have a city/town, county, and state rules to deal with, and you must research your local laws.
What are you intended purposes for the black powder?