If the barrel has never been fired, forget the brush. I would guess that you're just seeing colouration on your patches because either Butch's is reacting with the bronze brush, or else the brush is scraping a slight bit of the barrel steel away, which is wiped out with the patch.
mathman also makes a good point. If you're not cleaning the brushes between uses, you're just smearing the same crap back in the barrel for the patch to pick up.
Use Hoppes #9 with half a dozen patches, use the patch wrapped around the brush with JB paste for 30 strokes (use a bore guide!), one or two patches with Hoppes to rinse out the JB paste, then use 10-12 patches with Butch's. Should be down to bare metal at that point.
