One cold blue stands out in my experience. Van's. If you prep properly you can approximate a hot blue job with this stuff, and it wears well, doesn't smell, and tone can be controlled. We've refinished complete guns with this stuff and customers can't tell between it and a hot salt reblue. Again, preparation is the key and it helps to have a good blaster. Use silicon medium and you can actually produce a dark parkerized-appearing finish, athough it wears better than phosphate finishes because the stuff actually impregnates the steel.
If you look at the chemistry and the activity of blueing products on the market at the molecular level, many, if not most commercial "cold blues" are little more than paint. Study it for yourself, don't take my word.
Some gunshops will not like my honest report of actual experiences because they are selling this work as hot-blue work. No real problem, you'll be long dead before this blue job wears off. It's just a trade secret that many of us know. You'll be happy forever with the job and we (you, too) can do it for 1/2 hour in the blaster and other final prep, and 20 minutes with a clean toothbrush and Van's. Buff out, if you wish, it's in there.