Many slow rust bluing solutions contain copper sulphate which can plate out a thin wash of copper under some conditions. Laurel Mountain is one of the commercial solutions that contain copper sulphate, as do many of the recipes found in Angier's book.
This problem of copper plating out during rusting seems to happen more if the barrels are very warm during application of the rusting solution. I've also had a very thin wash of copper plate out on steel when acid pickling in hydrochloric acid that got contaminated with copper.
