Search down in this area a bit, and you'll find a recent series of messages on cold-bluing. They'll probablyl answer your questions. (Generally speaking, most of the folks here don't like to use cold-bluing for a complete gun... I've done it with good success, but uniformity of bluing is a problem. Also, I found Birchwood-Casey cold blue to be one of the most "temporary" of cold blues, while several brands from Brownells do quite well.)
You might consider Gun-kote or one of the other similar spray on/bake on finishes for your fiend's BP gun. Much, much more durable. You may have to create a "light" box to do the baking, however.