Black Lady was the result of Chibi Usabeing brainwashed by the Wiseman into believing no-one loved her. He used much negative energy and the Dark Crystal. The reason for the change of name could only be guessed, but my personal opinion is that when DiC dubbed the series, they created it for a younger audience, whom they didn't want to think that black was necessarily evil. Black lady fought against the senshi until Sailor Moon used her imperium silver crystal to become Neo Queen Serenity, then she persuaded Chibi Usa into believing that she was loved, in the future.
