I use 3 different darkening finishes, depending on steel, "brightness", and ability for the user to touch up.
birchwood-casey cold bluing gives an "old time" look. Brownell's oxpho-blue is amazing stuff, best cold blue ever.
the matte black is gunkote. Gunkote is a spray on, bake on finish. It's got some embedded moly super whatsit that really helps with moving parts, which we aren't so worried about. It's a good rust preventative, and you can epoxy over it. But you have to be fairly anal about following the curing directions!