and FYI, across the bridge directly north of the Black Dragon den is the Red Dragon Den, with 50 Red's for sale. There's also a quest right near the Black Den which has you retrieve Red Eggs from nearby the Red Den, after which you can purchase even more Red and Bone Dragons from the questgiver.
My little army of dragons is complete, accented with a nice little squad of Shamans
Too bad there aren't White or Blue dragons...