Black Site was a UNIT facility located at coordinates 51°30'47.7"N 0°08'51.9"W. It was situated at the site of the Time Fracture in Central London. (WC: 14681 UNIT Field Log, 14682 UNIT Field Log, 14683 UNIT Field Log, 14684 UNIT Field Log)
Behind the scenes
Though it's possible that a real world location doesn't exist in the same geographic space in the Doctor Who universe, such cases are few and far between. Thus, the map at right is probably a good indicator of the DWU location of Black Site.