Bluing is a chemical process which is pretty but doesn't really protect the metal very well. This is true even of the original rust bluing process. There are "cold bluing" products available from places like Brownells and MidwayUSA that are easier to apply, but are made mostly for local repair of blued firearms.
Parkerizing is a grayish finish created by dipping the parts in a hot phosphoric acid bath. It's also called a Phosphate finish and was first widely seen in WWII firearms. It's more durable than bluing, easier to perform, and does not require the perfect surface prep of bluing. There are industrial plating shops that can provide this finish - no idea what it costs for a bike frame.
But don't expect it to protect the bike too well. Firearms get very hot and the finish must be thin as clearances are small. They must also withstand bore cleaners which are very powerful solvents that attack copper and lead deposits in the bore - so imagine what they do to paint! But ask any GI stationed in Germany and they'll tell you a couple of days of rain will start making your bike look sad.
Firearms get coated with plenty of lubricant to protect them, especially for storage - parkerizing often has some "tooth" and helps hold the oil in place. Parkerized parts can rust. This is why you see so many new firearms that are stainless steel, polymer, or one of the new finishes like GunKote. Brownells has a FAQ at:
http://www.brownells.com/categories.aspx?c=4798
I looked into some of the spray-on gray finishes, but most of them require one hour in an oven at 350 degrees. There is a new one-part paint on finish called Stock-Kote. But it's really another Urethane paint system.
There are air-cured paint systems that give a similar look to parkerizing. Brownells Aluma-Hide II is an air-cure epoxy system available in Park Gray that goes on over bare metal from a spray can. They even have utube videos on how to apply them. It's about $10 per can.
Bikes don't face high temperatures, strong solvents and the finish can be quite thick. They're also a lot more accesible for the inevitable repairs. But a Aluma-Hide dark gray parkerized finish would be interesting.
