brownells works good.

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Of the Troops & For the Troops

I have been using the same bottle of "44/40" cold blue for decades, so I don't know if it's still made, but it was the best then, and still works fine. Make sure the spot is absolutely positively degreased.(use Berryman's B-12) Warm the spot with whatever heat source, not too hot to touch, just nice and warm. I spread the cold blue with a Q-tip. Let it dry, them buff it gently but firmly with 0000 steel wool. This might lighten it, requiring a second application, or a third. Just repeat the process until yer happy, then oil 'er up.

JLF

I use Birchwood Casey gun blue. It comes in a 3oz bottle or a touch-up felt tip pen. You can get both from Amazon for about ten bucks. It does a great job and a little goes a long way.

I've had good luck with the Birchwood Casey Patse. It's more like a gel, but it sticks in place better than their thin liquid to do the job. I have a bottle of Brownells Belgian blue that works well too, but for small areas, the paste is usually used. As has been said, degrease it very well and heat it up with a hear gun or hair dryer. Multiple applications are usually required to get a dark enough color. A little buffing with extra fine steel wool and a good oiling and you're good to go.

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https://facebook.com/M2bKydex/

Van's, applied to warm metal with a green 3M pad well saturated. Be sure to degrease thoroughly, and just keep rubbing gently until it's dark enough. Then wipe with alcohol on a paper towel and apply oil.

Lasts better than anything else I've tried.