A revolver is a pistol .They are both terms for any handgun .If your FAC says BP pistol or revolver they are talking about the same thing . So a 36 0r a 44 muzzle-loading revolver can be obtained .All propellant powder is black ,the difference lies in the make up of that propellant powder .If it's GUNPOWDER then you need a special license to buy ,carry and store it. If its de-natured nitrocellulose ,ie ,Pyrodex or 777 powder ,this can be bought and used in your pistol/revolver quite legally , in fact the police issuing authority in most cases would prefer you to use a gunpowder substitute as they are much safer than gunpowder (also called blackpowder or BP )
It's not rocket science ,it's easier to use Pyrodex /777 or other substitute ,it's far easier to buy and it's safer to keep at home.

I don't think I've missed anything out but there's sure to be someone ------