How does Blackhorn compare to 777 or Pyrodex as far as velocity achieved and cleanup?
Blackhorn has no sulfur content in any way or form, so it cleans just as easily as H777. Pyrodex would be a bit more messier because there are certain compounds in it like perchlorates which also happen to be the main constituent in corrosive ComBloc milsurp primers. (I was going to say "easily cleans up with water", but the fouling created by regular black powder also cleans up with plain water, if you know what you are doing)
As to power, Blackhorn claims to be more powerful than H777 but only in increased charges. In an inline loaded with a magnum load of 150 grains, Blackhorn will produce higher velocities than the same charge of H777, but in a cartridge with only 30-40 grains, I doubt Blackhorn will produce significantly higher energy than H777, Swiss, or Pyrodex.