"The wizards have found yet another source of troops, the dark barracks allow you to train mysterious troops made from the ever rare dark elixir." - Clash of Clans
Dark Barracks allow you to train troops using Dark Elixir. These troops, which include the Minion, Hog Rider, Valkyrie, Golem, and Witch, will be stationed in Army Camps. Like the Barracks, Dark Barracks are unable to function while being upgraded, considerably increasing the time it takes to increase your armies. This should be taken into consideration when deciding to upgrade a Dark Barracks.
You can speed up the training times of troops with gems, as well as with Boost which will speed up troop production for 45 minutes. The Boost option costs 25 gems and will increase the speed of the boosted Dark Barracks by 4 times. With the Christmas 2012 update, Barracks and now Dark Barracks doors glow when troop production is in progress thereby giving you a visual clue as to which Dark Barracks are producing troops and which are not.