i think they are a heavy canvas with wool insides. i have the Large white version.
they are very thick and for armour in themselves only. i wouldn't wear this under mail. i am in the process of closing the holes in the armpits cause i hate the un period holes. also, the padding in the inner elbow place will need to be removed. this will make the arm bend easier. I'm also on the fence about whether or not to take the straps/buckles off and point it closed with a long cord.
i wouldn't get black. i know it's your kit, black is in mine as well. keep the black for your surcoat if you wear one or your shield. but i would get the white/off white version.