Well I'm getting closer and closer to 70, 375 mining, and 360 blacksmithing, and the best use I've ever found for Dark Iron was to vendor a stack for a gold, I'm not sure how much use Dark iron was before BC because I only started playing this last December.
To sum up my dribble drabble, in my opinion Dark iron is useless, I hope someone that knows better can tell you how Dark iron was pre-BC