It is important to talk to Gloom'rel before any other dwarf since Doom'rel will begin the event to open the door which requires you to then kill Gloom'rel. Dead dwarves make poor teachers.
Unlike most smelting normal ore, it requires eight pieces of Dark Iron ore to create one bar of Dark Iron. Dark Iron can only be smelted at the Black Forge deep in the instance. You should take this into account, as some of the smithing recipes require as much as eighteen Dark Iron bars. That translates into at least one hundred and forty-four Dark Iron ore.
Also, Gloom'rel will remember that you paid him so if you ever decide to temporary drop mining as a skill then you can relearn the ability to smelt it again at a later time by simply paying him a visit.