I'd just ask at your local shop or range or club for someone who might want it, and give it to them (or trade). Someone who would pick it up in person, so there would be no hassles with shipping it.
There always someone nearby enough, who would enjoy a bit of the "holy black" or even Pyrodex as a gift or a fair trade.
Unlike smokeless powder, black powder is not a chemical compound, It is a chemical mixture, and does not degrade chemically over time. Even a century or more. (provided it is not physically scattered) Won't go off when wet, but can get wet and dry out with no harm done to it.
It is an explosive, and can be set off a number of ways you might not think would set it off, so its not as safe as Play-doh (or plastique
) but treated properly its not 'scary dangerous" if you know what you're doing.
With everything being in such demand today, it seems a shame to just waste it by spreading it on the lawn.
Which would be one safe way to dispose of it, if that's your choice...