Walter Grass blued the receiver and cut it back with abrasive, ending with fine emery powder on oiled steelwool. He refused to use acid due to danger of rusting. With the steelwool you just work until the color is nice and even. It really makes the engraving show up, and if you want some areas blue(black),just don't cut it back. Also not "real" French Grey but looks very nice and lasts well.
Mike