Black armor was actually very prevalent in European history. For example, the "Black Prince of Wales" was allegedly thus named for his black armor, although this is only one explanation and the name only came up after his death. In any way, that it is one possible explanation for the name already shows that black armor was indeed used and worn and is no artistic license.

Blackening is actually fairly easy and is done by oxidizing the steel, mostly by means of applying oil (a commonly available oil for this would have been linseed oil for example) to the armor and then heating it. This did not need a very intense heat to start an oxidizing process, about 300°C are enough.

With the right oil and technique, even a "cast iron" look (so really dark black) was achievable.