Don't do that in the future. Blackpowder is a very stable compound. It does not get unstable with age. Contact with water will ruin it, other than that, you could use Revolutionary War blackpowder if it was kept dry. (If anyone has genuine 1770's blackpowder, tell me your experience with shooting it!)
I occasionally come across stories of individuals who died attempting to defuse Civil War explosive shells. The American Civil war occurred before smokeless propellants & advanced explosives were developed for commercial and military uses. (I am sure someone had made some real unstable precursors to modern explosives by the 1860's) So the explosive shells of the era were loaded with black powder. Every so often someone digs one up, the shell body is still water proof and the black powder inside still good. And in the drilling, or whatever, the charge goes off.
I kept for decades a newspaper clipping of an antique shop in Gettysburg PA that exploded. The owner found a CW artillery shell and was drilling it out in the back of his shop. He died and so did a customer. Probably heat or a spark made its way to the main charge and Kaboom!
Artillery shells from WW1 are still killing French farmers. An unbelievable number of unexploded ordnance is still being pulled out of the ground. The propellants and explosives from that era, to today, become unstable with age. Moving old unexploded artillery shells is very risky. Making things more fun, chemical weapons from WW1 to after WW2 were just dumped in the ground, or at sea. During the WW1 period, both the British and the French buried many chemical shell dumps in place as that was the easiest thing to do. Blow up a chemical weapon and the fun is just starting!
More reading;
The Annual Iron Harvest of Unexploded Shells from the WWI battlefields
I'm not sure why somebody would be getting rid of black powder anyway. Smokeless, I could see, lot of potential unknowns. Much less so with BP.
