Blackhorn 209 in a flintlock would not be a good choice. Even if you duplexed the load. Flintlocks = black powder if you want good results. There is little use looking elsewhere for powders. Sure you can duplex the load but why? If you have to duplex with black powder, why not just use black powder. Black Powder is not that bad of a powder. It is consistent, powerful, and cleans up with simple soap and water.

People are always trying to find the better path with muzzleloaders. Less cleaning, less care, less rusting.. and that is good. But sometimes we just need to look at the rifle and the sport and realize, certain rifles need certain powders.