Mixing powders is a bad idea. It won't give you better groups. The problem you are having lies somewhere else. Either in the Ball/Patch combination or the lube, or something loose in the gun itself.
The problem with mixing powder is you don't know how much of each you are actually getting in your load, assuming you mix the powder is a horn, flask, or other container, before pouring it down a chamber or barrel. If you will use a chronograph to check velocities from randomly selected powder "drops", 5 or 10 tests shots, you will see the velocities all over the place. Now test each powder separately.
I found that screening black powder helped to reduce the Standard Deviation in Velocity. What you are proposing is the opposite. SDV is related to small group sizes. You can buy screen mesh through
McMasters.com. FFg mesh should be a minimum of 24 mesh; FFFg mesh should be a minimum of 46 mesh. I could only find 40 mesh at McMasters, but it will do. 4Fg mesh should be a minimum of 60 mesh screen.
I hope this helps. Ask a qualified BP gunsmith to take a look at your gun, and test fire it to see what is going on. If you know an experienced shooter, who is a very good shot, ask him to shoot the gun and see if he gets the same results. Ask an experienced MLer to watch you clean and load your gun for the next shot, and then watch you shoot. He probably will be able to spot things you are doing differently than you did with that first shot out of a clean barrel. :thumbsup: