But... black is the absence of colors...!
Yes and same is "white" so why do we have "white tunic over mail" and "white tunic"...HUH?!
Erm...white is not absence of colours but all colours mixed together Alex^^ So it's ok.
Btw, I'm against adding something like black tunics. Colours mostly used in medieval times were red, blue, green, yellow, brown and all the other natural colours that can easily be made out of plants.