I have an old "shooter" Luger, with no collector's value. I've stripped it down to white metal (using the Birchwood Casey rust and blue remover) and then reblued it using Brownell's Ox-Pho Blue. It took a while, with the bluing compound applied in very thin coats (using an almost-dry patch), but it now looks like it had never been reblued, with a nice dark blue (almost black) finish.
Ox-Pho Blue, which was recommended to me by George Stringer, moderator of TFL's "The Smithy" forum, is the only cold blue I've ever really had success with, although I've found that using several other brands in conjunction with Ox-Pho blue gives very nice results, when "patching" or "repairing" bad spots.
I bought a BIG bottle, but you can probably get buy with a small one. Most of the stuff you'll find in gunshops and sporting goods stores simply won't work... or will come off the first time you clean the gun.