[Krieg]

Blueing is a process in which oxidizing chemicals are added to the steel to create a protective coating. It is basically black-blue coloured oxide of iron.

There are chemicals you can buy to blue your gun, this is cold blueing.

Also there is hot blueing, where steel parts are dipped in a solution of boiling water and nitrates, chromates.

BTW that Beretta really gives a good impression of blueing.