You can't go wrong trying different charges. I can't contribute much with regard to BlackHorn because I have very little experience with it. If you have some Triple 7 or Pyrodex handy, I would give it a try with that bullet for a point of comparison.

As you know, tight groups are a combination of gun, load and shooting technique. I assume you're shooting from a good solid rest, so here are the first couple of things I would be asking myself. Have I shot this load enough to know for sure this is the best it will do, or could I have just had a less than stellar day shooting? Has this gun shot some other load/bullet/powder really well? If so, what was the bullet weight/powder charge?