Hot salt bluing is the only true bluing, anything else is paint or dye. True hot salt blue is not a chemical finish, it's an oxide coating formed by the reaction between the metal itself and the specific salts in the bluing solution, it is essentially rust under highly controlled conditions. It's not for the faint of heart to do the process at the hobby or DIY level. The metal salts used in bluing are nasty corrosive and poisonous, as are the vapors. De-greaser and the de-greasing process is almost as bad. Bluing requires lots of dedicated space and effective ventilation. It requires pans or vessels that will allow all parts to be submerged completely, and a method to heat and maintain the temperature of the solutions and rinse vats. Cost of the service is variable and dependent on a few factors. Much akin to a good paint job, a good bluing job begins with the preparation which is labor intensive, the poorer the condition, the more labor. Also, the type of final finish you order will have the most affect on the final price. A high polish will be the most costly, buffing blue to a high polish is an art, and not for the novice. If you make a mistake polishing, that part has to go back to the beginning of the process and start over. Since it is such an involved process, you will often be looking at long turnaround times. The gunsmiths i worked for would only set up to blue a few times a year once they had 10 or 12 jobs to blue. I don't miss bluing, at all.