The Pyrodex is fine but you won't see real black on a shelf, you have to ask for it. The spout should unscrew from the flask and get a small funnel for filling. Real bp will go through a fairly small hole but Pyro is bulkier so it depends on just how small the hole is. Make sure there's no oil left in the chambers before you load it or you will be doing some major cussing.
You might even want to pop a cap on all six before loading.