Black Lady was the form Chibiusa took when she was turned to evil by Wiseman.
In the anime, Chibiusa travelled to the future with the Sailor Senshi and once there, heard King Endymion tell them that the Silver Crystal disappeared shortly before the Black Moon Clan attacked.
Thinking that it was her fault that the Crystal vanished, and thus her mother was hurt, she became upset and ran away, leaving the shield around the Crystal Palace. Wiseman found her there and began showing her illusions that made her think that everyone blamed her for what happened. He told her that he could convince...