I wouldn't hesitate to shoot a bullet/Blackhorn load that's been sitting in the bore for a few years any more than I would hesitate to shoot powder that's been stored in a container for years. The powder is either good or weakened, but not increased in power. Just be sure to check the bore to make sure the bullet is fully seated and everything is clean from the bullet to the muzzle. If the powder is so weakened that the bullet does not exit the muzzle (unlikely) pull the breech plug and push everything out.