Black Cells

The Black Cells is one level of the dungeons of the Red Keep. They are reserved for prisoners accused of high crimes, such as treason. Cells are completely dark.

Varys has access to the Black Cells, but dons the guise of a gaoler to enter.

Season 1
Ned Stark is confined to Black Cells following his arrest. He is visited twice by Varys who seeks to convince him to confess treason in order to prevent war and to safe the life of his daughter Sansa.

Season 2
After learning that Pycelle is an informant for Queen Cersei and hearing him confess that he allowed Jon Arryn to perish, Tyrion Lannister has his men Timett and Bronn cut off Pycelle's beard and throw him into one of the Black Cells.

In the books
In the A Song of Ice and Fire books the Black Cells are the third level of the Red Keep's dungeons. On the upper level are cells with high narrow windows where common criminals are confined together. The second level has smaller, personal cells without windows for highborn captives. Torches in the halls cast light through the bars. The Black Cells themselves are smaller still and have doors of wood so that no light enters them; they are reserved for the most vile and dangerous prisoners. The fourth level is the one used for torture and noone who enters leaves alive.