Cold blues are not very durable, and I suspect you will not be pleased with the result. Additionally, they contain acids which can initiate surface rust and weaken springs over time if they aren't properly neutralized, so you will want to strip the bolt down to apply the stuff.
If you are up to stripping the bolt, then I would suggest you look at the old fashioned process of rust bluing. I made a front sight extension for my old Ruger Mk I bull barrel pistol by filing it from key stock. I rust blued it and got very satisfactory matching appearance and ruggedness. The process consists of using a special solution (such as the Pilkington solution sold by Brownells) to intentionally induce light surface rust that is then boiled in distilled or deionized water to convert it to the black oxide form. After boiling, the loose black oxide is rubbed off (carded) with degreased steel wool or a fine wire brush. The process is repeated a half dozen times, at the end of which you have a beautiful looking bluing job that is real bluing and not a selenium substitute, as with cold blues. The solution that induces rust needs about 24 hours to act, so this a week-long project.