You can learn Smelt: Dark Iron from the ghostly dwarf Gloom'Rel in the Seven Dwarves room in the instance Blackrock Depths. You must talk to him before you start the script in that room, if you're not the group leader let the leader know that you need a moment before the script is started.
The price Gloom'Rel asks is 10 Truesilver Bars, 20 Gold bars and 2 star rubies.