Bright blue is prettier, but a phosphate finish will hold up better on a working gun. Blue is awfully 'thin' and wears through quickly, plus it seems less resistant to rust overall to me. If you plan on using a blued gun in harsh conditions, multiple light coats of a good hard wax (carnauba, johnsons paste wax etc) will help preserve the finish some. That means wood and metal both, I usually pull off the furniture on a new-to-me gun and heavily coat the unfinished ends/insides with wax to seal the grain and protect the metal it comes in contact with. Just leave a coat of wax on the gun and let it dry without polishing before you go out, it will dull the bright finish some and will polish right off when you want the gun shiny again.
Look in some of the tool catalogs etc and see what wax they offer for woodworkers to use on saw blades and other tools- those will work too.
lpl