Depends on which period. In 1403 plate armour was still in a transition phase and was also still more expensive than mail, meaning that the type of blokes who would be able to afford plate armour would also be able to afford "pretty" polished armours (blackened armour really wasn't in fashion until later with some German styles and even later with the decorated styles of 'black and white 'armour in the 16th century).

At the end of the 15th century when plate started to get easier to produce (less labour intensive, so cheaper) we started to see 'munutions grade' plate armour, that was mass produced and in large batches and no longer specialized. Around this period more unpolished black-ish armour was seen. There are a few recorded cases of knights wearing black armour in the game's timeframe, such as Zawisza Czarny, but I'd generally consider it rare and perhaps too rare for it worth the modelling effort to add to the game.