The National Fire Protection Association (NFPA) classifies smokeless powder as a "propellant," but black powder is classified as an explosive. It blows up, it doesn't burn
The main difference between black powder and smokeless powder is that black powder, once it has been grained and corned, will possess a FIXED burn rate. That means whether the powder is confined to a cartridge casing, a bag as in an artillery charge, or spread out along the ground to form a fuse, it will burn at the same rate. That is why during the manufacture of a batch of black powder, the corning process is the most important. The percentage of nitrate in the powder determines the total energy yield of the powder. The corning/screening determines how fast this energy will be released. And that is what makes black powder especially dangerous. Because even if a very large vat of black powder is left in an open space, with no cover or obstruction on top of it, and it makes contact with an ignition source, it will still deflagrate violently, with the speed and intensity of an explosion. After all, to the flesh and blood human body, it does not matter whether we get hit with a 3000 feet/second blast wave or 10,000 feet/second blast wave. We are equally dead if we happen to be in such a situation. The deflagration of the powder inside the vat will be the same as if it is confined inside a cartridge, because once the powder is corned, it's rate of combustion has already been fixed. The finer the grains, the more rapid the rate of combustion. Much more explosive energy. That is why anything below FFF CANNOT be used in firearms except as a pan primer for flintlocks or a burn spreader at the very bottom of a cartridge case, only a few grains with even distribution. The bigger the grains, the more controlled the burn rate. Before the adoption of smokeless propellant, the main powder for large-caliber naval guns was called Hexagonal. It was developed in Germany and was a sulfur-free nitrate/willow mixture that was corned into pellets the size of large washers.
Smokeless powder, on the other hand, is a compression-controlled propellant. The more pressure is placed on a charge of powder, the faster it burns. Rifle powders inside a cartridge that can propel little .22 slugs up to 4000 feet/sec, when spread out on a cinderblock, will only burn like a paraffin firestarter. An open vat of smokeless catching fire will produce an energetic, rocket-like conflagration with a lot of foul-smelling smoke, but the effects would not be nearly as destructive as an open vat of black powder being ignited. And when smokeless powder has been compressed to a certain pressure, IT WILL detonate. Just like a plastic explosive.
Hope this clarifies the difference between black and smokeless powders.
I have made my own black powder in the past, in the form of 80/20 nitrate/charcoal concentration without sulfur so I have considerable experience with the subject. Sulfur-free black powder is virtually smokeless when fired and the 80/20 combination will drive a 220-grain lead conical out of a full-power .45 Long Colt casing at 1200 feet/second out of an 8-inch barreled revolver. This information is for educational purposes ONLY.