Well, there are significant differences between TNT and black powder.
TNT is a high explosive, while black powder is a low explosive. The high/low designation is determined by the propagation characteristics of the explosive's shock wave and its burning rate.
As for compressed black powder being flammable, I don't think that is correct at all.
Black powder burns/explodes, which ever you like, at the same rate whether it's compressed or not.
Smokeless powder is, uncompressed, a flammable solid, and thus can be shipped on trucks with "flammable solid" plackards.
Black powder, however, in any bulk form, can only be shipped on trucks with "Explosive" plackards.