Pyrodex was developed in the early 1970's to negate some of the dangerous features of black powder, esp. black powder's propensity to ignite and explode unless it is treated with respect in regard to proper handling. It is interchangeable with black on a volume to volume basis, not on a weight basis. Because it is harder to ignite than black, pyrodex is best suited to percussion and cartridge weapons. It requires thorough clean-up as it is in my opinion more corrosive than black.
Just what pyrodex is, I don't know and I don't know if that is still a secret.