How do you get smelt dark iron in World of Warcraft?

You go into Blackrock Depths, where you will find a room with 7 Dwarves - the Tribunal of Ages. One of them is called Gloom'rel. You can talk to him, and he will offer to teach you this skill in return for the following materials (bring them when you go in, it's much faster):

• 2 Star rubies
• 10 Truesilver bars
• 20 Gold bars

He will tell you to put it in the Spectral Chalice, by clicking on it. After you've done that, talk to him again and you learn the skill.