Black Void

The Black Void was a cosmic location outside of the universe. It may have had some connection to the Land of Fiction and the White Void located "outside" of the Land, which the Second Doctor stated was "nowhere."

When 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 was 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 travelers and the Master faded back into the black void where the TARDIS came back together. (TV: The Mind Robber)

Behind the scenes
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."