Medieval armour technique short: How to Blue Armour

How to blue armour.
Bluing armour is not the great mystery that it first appears.
By applying a consistent heat across the surface of a polished piece of material you will cause it to heat up. As the material passes through a certain temperature range it will change colour.
At about 280C (540F) the material will reach a blue/purple colour. By freezing this in oil you can preserve the colour on the surface of the material, this helps resist rust (a little bit) and has an aesthetic quality about it.
You can blue using chemicals, or even baking a piece in an oven.
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For how to blacken armour, take a look here:
Bluing is a great process for accent pieces of armour or even the whole piece. It is scratch resistant but will eventually be rubbed or scratched away.