Black was a colour.
The Master usually dressed in all-black clothes. (TV: Terror of the Autons [additional sources needed]) The Trickster normally wore a long black hooded robe and gloves. (TV: Whatever Happened to Sarah Jane? [additional sources needed])
The Host of the Lupine Wavelength Haemovariform had all-black eyes. (TV: Tooth and Claw) Grace Holloway's eyes turned all-black when she was controlled by the Master. (TV: Doctor Who) Mercy Hartigan's eyes turned all-black when she was converted into the CyberKing. (TV: The Next Doctor) Owen Harper's eyes turned all-black when he was possessed by Death. (TV: Dead Man Walking)