To get the recipe to make Dark Iron bars you’ll need to get the following materials:
2x Star Ruby
20x Gold Bar
10x Truesilver Bar
Then head to Blackrock Depths in Blackrock Mountain in the Eastern Kingdoms.
You’ll have to pick up a quest called The Spectral Chalice, which you can find by taking the Mole Machine near the entrance to the dungeon. Choose “Just past the Grim Guzzler” and take the door on the right. You’ll need to make your way to “The Chamber of the Seven” and then talk to Gloom’rel.
Pick up the quest by talking to him and turn in the materials. After you have the recipe, you’ll need to fight the boss in this room to get access to the forge needed to make the bars.
For Dark Iron Bars, eight ore = one bar.