The Black Hand was a title of Morgoth, used by Beleg to explain how Morgoth's power had expanded southwards Beleriand after the Nirnaeth Arnoediad.[1]
In the Third Age, Gollum twice applied the same epithet to Sauron's left hand. Gollum mentioned how Sauron had just four fingers on his "black hand", this being of course due to the fact that Isildur had cut off the finger that bore the One Ring.[2]