Rockdoc,
The "billions of pits" in the surface of rustblued barrels, are the result of the bluing solution.The rusting is killed by boiling and the color is changed to black at the same time.It is actually these pits that give rustblue it's famed wear resistance( wear would have to reach the bottom of the pits).This "pitting"("biting")is also the reason that polishing with finer than worn 320 grit abrasive is futile.
Mike