That IS strange. Sounds almost like a load with no powder, and just the primer went off enough to lodge the bullet in the barrel. The powder color just doesn't make sense. It must have had some kind of ignition to make it turn color, I'd think, and I really don't understand why all the other shells you pulled the bullets from had propre colored powder and it worked on the successive reloadings.
I had a powder experience with my 45/70 this summer. When using factory (Rem.) 300 gr. ammo to get it sighted in with and get some casings to start loading with, I noticed powder granules sticking to the patch when I cleaned the bore after each shot during the break-in. I tapped the casings on the bench, and a few more particles fell out. It was not the same color as the factory powder, as I cut a factory shell apart when I got to the house. This happened every shot with the factory ammo. I also reloaded with H4198 and have not had that happen since, but this is not the same scenario you describe......strange.... Hope someone on here has a better idea.....