Keep in mind that you have tolerances on guns with moving parts that are in thousands of an inch and must be held for functionality. Coatings such as paint whether they be polymers, epoxies, anodized or powder coating are by there nature at min .006 - .008" and I mean the least. The process of prepping and applying mean machined surfaces must be etched or profiled for adhesion and applicaiton must be tailored to maintain breaks or lines where the coating does not interfere with the slide, cylinder mechanism or whatever riding surfaces.
Bluing works because it can be applied without all the crap noted above and because the process can be customized for color and appearance such as early Colts and S&W's. Finally bluing yield an excellent surface for lubricity an element absolutely necessary for guns to function