why are scrying mirrors black? and curved?

you can use a plain old bowl of water if you want to
but the black helps to stop reflections though … so all I do is put black cloth below the bowl

It’s to help us see our inner self, not our outer self.

Black absorbs light and reflections so that you mind only focuses on staring into dark and not be distracted by your own reflection or something in the room.
Pangel has a good suggestion. Either a clear bowl with black under it, or water with ink poured in. Take a picture frame and spray paint once side with black paint. Put it back into the frame (a very simple frame) with the painted side inside.
Go for it.

you can use many things to scry. Black, concaved, with water is usually the best because it leads to less distractions. Keeps you focused where a regular mirror and the like hold too many distractions. (Personal Experience)