The Black Void was a cosmic location outside of the universe.
The universe was constantly expanding into the black void that existed before Event One, and on the edge of the universe one could see quasars and other forms of cosmological expansion happening. The Monk summoned Artemis the Chronovore at the edge of the black void. (PROSE: No Future) Additionally, the Dark existed in the void before the Big Bang. (PROSE: Fear of the Dark) Later TARDIS models were able to traverse the boundries of the universe, enter the eternal nothingness and return. (PROSE: Cat's Cradle: Witch Mark)
The Watcher took the Doctor's TARDIS out of space and time, from which Adric and Nyssa would be safe and could observe the entropy wave the Master had unleashed following the destruction of Logopolis. (TV: Logopolis)
After arriving in the White Void located "outside" of the Land of Fiction, which the Second Doctor stated was "nowhere" and outside of time and space, an intelligence believed to be the Master of the Land attacked the Doctor's mind as well as his companions Jamie McCrimmon and Zoe Heriot. The TARDIS relocated to the black void, where it turned over and over before losing its dimensional transcendence and breaking apart as six sides of a police box. The Doctor drifted away from Jamie and Zoe, who held onto the console before being engulfed in smoke. They eventually arrived in the Land. After defeating the Master Brain and causing the Land to dissolve, the TARDIS travellers and the Master faded back into the black void where the TARDIS came back together. (TV: The Mind Robber)
Behind the scenes Edit
It has been noted that, although the Doctor claims the White Void as being "nowhere," the fact that it's white suggests the existence of photons, in which case the Black Void would truly be "nowhere."