Sometimes it is better to leave them as is but if you must I will tell you what I do and it works very well...
Clean whatever part it is thoroughly with alcohol and let it dry.Do not touch any surface and be sure there is absolutely no oil on the metal to be blued.
I use 44/40 paste as you wipe it on and wipe it off and it leaves a durable dark finish, though it is more likely to be black than blue.It cleans up the looks of an old gun very well and is easy to use...