It is confirmed that Black Citadel and the Black Temple are one and the same. As explanation, it has been previously stated that Magtheridon ruled Outland from the "Black Citadel," his seat of power where he eventually would be confronted by Illidan (see sources below). The linked article specifically states that Illidan confronted Magtheridon at the "Black Temple," which clearly indicates the two are the same place, and furthermore indicates that Magtheridon was then taken to Hellfire Citadel, clearly indicating that as a separate location.