OP. If the rust is till on the blade, i would try this: http://www.gunzilla.us/
Sprat it on, and let it sit fro 24 hours. Wipe it off, and see if it needs a second application. If it does, repeat step one, and after the second application spray some on the blade, and scrub it with a nylon bristle brush. If it will take rust off of a blued barrel without damaging the blueing, it should work on the coating on that blade.