Out of curiosity, I have cleaned new barrels which had never seen powder or bullet with both J&B and IOSSO. You are correct
patches will come out black. Prove this to yourself, on the outside of a barrel. Some very bad factory barrels in hunting calibers
which I have cleaned come out clean ( using a borescope) with as few as 10 strokes using J&B or iosso on a brass brush. It
works, the question is, Is this excessive for a good match barrel, and does a match barrel need to be taken down to
bare metal with each cleaning, or just held at a light level.