The Black Guardian was a conceptual entity embodying Darkness and Chaos. He used the Shadow to stop the Fourth Doctor from claiming Princess Astra of Atrios, the sixth segment of the Key to Time. 

After the Doctor defeated the Shadow, he assembled  the Key and outwitted the Black Guardian, enabling the White Guardian to restore the universal balance (5F). 

The Black Guardian swore revenge against the Doctor and later, used Turlough to try to kill the Fifth Doctor (6F), (6G). The White Guardian and the Black Guardian offered Enlightenment, symbolized

by a crystal of unknown powers and great value, as prize to the winner of a planet spanning race undertaken by the Eternals. 

The Black Guardian hoped that, with such a prize, the Eternals might wreak havoc throughout the Universe. Instead, the Fifth Doctor won the race but turned down the Enlightenment, which was offered to Turlough. 

The Black Guardian reminded Turlough that, to get the Crystal, he had to kill the Doctor. But Turlough refused and threw the crystal at the Black Guardian, who vanished in a burst of flames. 

However, the White Guardian explained that the Black Guardian would exist as long as he did, and would continue to do so until neither were needed any longer (6H).