Black powder was often used by the proof houses of various countries in the early years for proofing barrels which would be stamped with Smokeless Proof marks. As mentiones a fine grained black could be used to give the desired maximum proof pressure for the chamber. Thus for that purpose it mattered not which powder was used. The black however maintained a higher pressure for the rest of the barrel so was actually a more severe test than a smokeless charge giving the same maximum pressure. Later on special smokeless proof powders were developed which did away with the need for using black. As I recall the French marks for double, triple etc proof varied over over time. One has to correlate them with the point in time the gun in question was proofed.
