There's many techniques to give a blue (or other) colour to a polished steel surface (you may be familiar with the term "bluing"). The basis of all of them is the formation of a (usually thin) layer of specific corrosion products that feature the colour of choice. This, of course, is different than painting, enameling, lacquering or other "covering" techniques, in that the idea is making the metal react chemically and produce a specific spices of compound with the right coloration. Depending on their nature, many of these layers have also some more or less effective protective properties in front of oxidation, combining aesthetics with functionality.
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