A
tool utilized for
scrying, usually made of
obsidian, but also may be
glass with the
reverse side painted
black.
John Dee and Edward Kelly used a Black Mirror (and also the more famous Crystal Ball) in their communications with Angels.
Nostradamus used a similar technique of scrying by peering into bowl of water in which ink had been placed. The idea is too 'look through' the darkness as if it were a window and watch the 'vision' unfold.
Black Mirrors are also to be found amongst the tools of the magician in the western esoteric tradition.