I agree with Dustin (LeFusil) about the use of products that contain acids, and that includes every product mentioned here so far. They can all create frosting or micro-pitting of steel, depending upon the concentration and length of application. Bluing is a form of rust or oxidation, and acids will certainly remove it, but any micro-pitting or frosting requires additional metal to be removed by abrasive polishing.
For the last several years, my rust and blue remover of choice has been a solution of feed store molasses and water. I use dried molasses at a ratio of about 5 lbs. to 5 gallons of water, but this ratio is not critical. It is slow, taking up to a week or two in warm weather, but it is effective, dirt-cheap, non-toxic, and can be reused many times before it loses effectiveness. It gets a little bit smelly as it ferments, but not near as bad as some folks say. It is a chelating rust remover like Evaporust, and requires complete submersion of the parts. The oxides turn into a black slime that is easily scrubbed off with water, and rubber gloves are a good idea. What I really like is that it does not attack sound steel or iron. It only removes the rust. And then I use abrasive paper or emery cloth for final polishing. I have put rusty tools, etc. in my bucket of molasses solution and forgotten about them for several months, and the extended soak caused no damage. Some people say it will cause pitting of machined surfaces but I don't believe them. If there is pitting after rust removal, it was there all along, hidden under the rust, and removing the rust is not going to restore pre-existing pitting. All that said, I probably wouldn't buy a 50 lb. sack of molasses just to remove the bluing on a trigger guard and floor plate.
