I used Birchwood Casey Super Blue. Application is easy, just follow the instructions. It blues fairly fast. I wanted a worn blue heading to gray appearance, so I rubbed it back with 0000 steel wool. To get more of a gray, I probably should have used something like a scotch bite pad. I corresponded with a builder who built a North Carolina Mountain rifle. He put an aged gray on the metal by using Super Blue and and a scotch bite pad. He let the blue work on the metal over night to get some nice pitting before he rubbed it back. Beautiful gun. Look at some of the threads from Mike Brooks. His work is an inspiration. I believe he uses LMF to brown the barrel, takes it all back, then Super Blue and rubs it back with steel wool to a gray. You can do pretty much whatever you want. I think that the cold blue and cold browing works well for an aged apperance. I haven't tried using these applications to try for a deep "factory" blue.
Scott