You won't find out how good / how contaminated that old jug of Pyrodex is, not until you shoot it. Once it does get moisture-contaminated thou, it loses power-punch and ability to ignite more easily.
That Goex given to you is a different creature. It (real blackpowder) can recover 100% from getting moisture in it.
(in my opinion) Real Black is a better / smarter purchase than Pyrodex, which rusts bores quicker than Real Black. Any real blackpowqder ignites more easily than Pyrodex. To clarify that statement, Pyrodex pellets have real blackpowder on the ignition-side of their pellets, to enhance ignition.