Black was a colour.
The Master usually dressed in mostly black clothes. (TV: Terror of the Autons, The Deadly Assassin, The Keeper of Traken, Doctor Who, Last of the Time Lords, The Doctor Falls) The Trickster normally wore a long black hooded robe and gloves. (TV: Whatever Happened to Sarah Jane?, The Temptation of Sarah Jane Smith, The Wedding of Sarah Jane Smith)
The Ninth Doctor normally wore all black articles of clothing, save for a long sleeve jumper of a different colour, (TV: Rose et al.) though by the end of his life even the jumper he wore was black. (TV: Bad Wolf/The Parting of the Ways)
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)
In Through the Eye of Eternity, the Black Eye of Eternity generally symbolised that the week's episode would be a black comedy. One fan wearing a black Eye pin, an apprentice mortician in Shoestring, Indiana, was fired after laughing at a client's somber remark. (PROSE: The Book of the War)