Over the years I have cold blued many guns, mostly cheap 22 rf and old bouble barrel shotguns. I just finished a really beatup 86 Winchester in 40-82.
What works best for me is Oxpho-blue cream, Clean metal and warm to the touch, not hot. Apply generous coat of the cream and let set longer than the instructions say. Wipe done very vigerously and then go over it, burnish as they say but with the regular Oxpho-blue liquid and put it on with small pieces of 0000 steel wool. As soon as the wool gets black turn it over to a clean piece. I cut 8 pieces out of a pad and will generally use 2 pads to a job. A short while ago I did the best one ever, wish I could do it everytime. A friend had me rebarrel and old 98 Mauser, I'm serious, using the above method you could not tell it from a factory matt finish. Type of steel and type of heat treat makes a big difference sometimes