Cold blue works, but it's not a single application. Remember it's coloring the steel, it's not a finish. The barrel / receiver needs to be bright and shiny.
The more you prep, the better the result.
Degrease, then do not touch the metal with you fingers again. Any bit of oil won't let the blue work. You'll get a spotty finish. Apply the blue, let it sit a minute or two to dry, wipe off any with a clean paper towel. Repeat, repeat, repeat. When done buff with that 6xO steel wool, if it's not dark enough, keep repeating.