While it's true most cold blues are sold as touch ups, Brownells sells a couple that work very well indeed on entire weapons. Not as durable as some other blues but a bunch cheaper.. 'Dicropan IM' is a very good bluing agent but requires more work than my favorite, 'Oxpho Blue'. Oxpho Blue requires less trouble than other cold blue products. It will blue thru oil. Not a massive coat but the steel does NOT have to be oil/grease free to color nicely. It does not produce a 'black' color but rather a dense dark blue, on most steel. Oxpho blue is the most abrasion resistant and easiest to apply cold blue I've ever used. It will absolutely do an entire firearm. Perhaps not a fine slow rust blue but quite cost effective.
