Tell you the truth...I had been tempted when Blackhorn first appeared on the market.
However, like Hawg said, you should not. It is designed only for use in inline muzzleloaders.
Blackhorn will only give you much powerful results when a large amount is used, as in an inline where "magnum" charges of up to 150 grains are commonly used. Even if you are able to get ignition in a pistol, I doubt 30-40 grains of that stuff will be any different than the same amount of H777 or Pyrodex.
And cartridges? Well, cartridge primers are much more powerful and produce a lot more energy than a standard percussion cap. So for use in cartridges, it would be reasonable. But then, unless all I have is Blackhorn on hand, I think I would use regular smokeless powder or black powder for my cartridges.