Black powder is an explosive material for purposes of federal explosives laws and regulations.
However, the law exempts from regulation commercially manufactured black powder in quantities not exceeding 50 pounds (as well as percussion caps, safety and pyrotechnic fuses, quills, quick and slow matches, and friction primers) intended to be used solely for sporting, recreational, or cultural purposes in antique firearms as defined in 18 U.S.C. 921(a)(16) or in antique devices exempted from the term "destructive device" in 18 U.S.C. 921(a)(4).
Regardless, persons engaged in the business of importing, manufacturing, or dealing in black powder in any quantity must have a federal explosives license, and comply with recordkeeping, storage and conduct of business requirements.
[18 U.S.C. 841(c), 841(d), 845(a)(5); 27 CFR 555.11: definitions of "explosives" and "explosive materials", 555.141(b)]