If you can mine dark iron, you should be high enough level to learn to smelt it. The trick is finding a group that is willing to fight all the way to the end of blackrock depths. There is a room with seven ghost dwarves which block access to the final section - one of them is Gloom'rel and you need to talk to him before starting the fight script. He will tell you that sacrifices must be made in order to learn to smelt dark iron (or something of that sort) and a ghostly chalice appears next to him. Click on the chalice, give up your gold, truesilver, and rubies, and you have the skill.
The Black Iron Forge, which is the only place that dark iron can be smelted, is just beyond the ghost room and to the right. There is a bridge guarded by fire elementals that leads to the molten core instance, and a tiny pathway over the lava river on the left that leads to the forge, guarded by a single fire elemental. Kill it, walk out to the forge, and smelt down your ore. Remember it takes eight ore to make one bar of dark iron - you have to have a lot of carrying space to get a useful quantity of dark iron out of a trip.
Good luck!