The more I shoot real black powder, the more I appreciate just how terrible it is not.
I haven't tried BH209 in a muzzleloader because I don't have a muzzleloader that uses a sealed 209 ignition. I have used it in cartridges and it works well.
Even if your rifle does use 209 primers, it may not work. Some breech plugs have a flash hole that's too tiny and long for good ignition with BH209 I hear. When the flash hole is tiny and long, a 209 primer essentially becomes the same as a #11 percussion cap.
My brother has one of those muzzleloaders that uses sealed 209 ignition and he loves the stuff because he can shoot very small groups with it.