Source: Trade Skill Sticky
Section: Mining
Resource:
Jukaar's Mining Guide (note, this is also in the guide section on the left).
Quote:
Dark Iron - In Blackrock Depths and Molten Core, deposits can be found in any of the numerous caverns.
Dark Iron serves 2 purposes. You can either smelt it to create bars for any of the Blackrock Depths exclusive recipes, or you can turn it in to Lokhtos Darkbargainer in the bar in Blackrock Depths to increase your Thorium Brotherhood faction. When you get your Thorium Brotherhood faction to Honored and above, Lokhtos will sell you some really nice crafting recipes. Before you get too excited about that prospect, read the paragraph below about smelting Dark Iron Ore.
To create one Dark Iron Bar, you need 8 Dark Iron Ore, and you can only smelt Dark Iron at the Dark Forge, which is located near the portal to Molten Core. Before you can smelt it, you need to learn how to smelt Dark Iron Ore from Gloom'rel deep in Blackrock Depths in the 7 Ghosts room. He'll teach you how to smelt it if you give him 20 Gold Bars, 10 Truesilver Bars, and 2 Star Rubies. You can only make items that require Dark Iron Bars at the Black Anvil, which is on the other side of the instance where Lord Incendius is located.
Hope that helps!