Most Fire Marshals (yours may differ) use the codes and standards of the National Fire Protection Assoc (NFPA) for powder storage requirements. And, the NFPA standards are the ones typically referenced in the powder manufacturers' literature. The standards are different for smokeless and black powder. The linked site, to your box, makes no reference to black powder storage.
From the NFPA 495-41 & 42, 2010 Edition:
Chapter 14 Small Arms Ammunition and Primers, Smokeless Propellants, and Black Powder Propellants
14.4 Black Powder
14.4.3 Black Powder intended for personal use in quantities not exceeding 9.1 kg (20 lb) shall be permitted to be stored in residences where kept in the original containers and stored in a wooden box or cabinet having walls of at least a 25.4 mm (1 in.) nominal thickness.