The Blood Furnace is one of the dungeons in Burning Crusade Classic. It can be accessed from level 58.
The entrance is located on the central platform of Hellfire Citadel, in the heart of the Hellfire Peninsula in Outland.
Location
To access Heroic difficulty, you must be level 70 and have the Flamewrought Key, which is available to players who have attained Revered reputation with Honor Hold (Alliance) or Thrallmar (Horde).