Cold bluing is a passivation process used to partially protect steel against rust using a black oxide coating). It is named after the blue-black appearance of the oxide coating. Cold bluing is generally a selenium dioxide-based compound that colors steel black or a very dark grey). It is a difficult product to apply evenly, offers minimal protection, and is generally best used for small fast repair jobs and touch-ups). Cold bluing is not particularly resistant to holster wear, nor does it provide a large degree of rust resistance).

The cold bluing process involves cleaning, polishing, degreasing, and drying the gun parts before applying the cold-bluing solution. After 12 hours, the rust can be scrubbed off the gun and the bluing solution reapplied. The cleaning and bluing process is repeated every 12 hours until the gun is restored to satisfaction. Cold bluing is not as durable as hot bluing because friction against fabrics or other materials can rub it off.

Cold bluing is usually used for small touch-ups to a guns finish to prevent a small scratch from becoming a major source of rust on a gun over time). It is not recommended to use cold bluing as a touch-up where friction is present). Hot bluing is the best option for steel and stainless steel guns.