Flitz is good if used sparingly and with very little pressure, try to wipe it off with a good micro fiber cloth. I wouldn't use the Mothers as I believe it to be more abrasive. Mothers works very well on stainless guns, in fact with a micro fiber cloth you can get an almost mirror bright shine
Take your time on blued guns as was mentioned earlier, the finish is indeed very thin in fact I always start polishing on the bottom of the barrel in an inconspicuous area. Done properly the difference on a blued gun can be amazing.