Step 1:
First, find yourself a group to UBRS that will do the whole place. Travel to Blackrock Mountain and run up along the chain that leads to BRD and jump down there, turn right and run towards some kind of place, jump on to it and your just outside the entrance to BRS. Get some guy who can help you kill the Scarshield Quartermaster. Start killing the mobs to the right and fight them until you reach the Scarshield Quartermaster, kill him and loot the letter he drops. Rightclick the letter and start the quest, Blackhand's Command.
Step 2:
Now go along with your group until you reach General Drakkisath, kill him and then touch the orb behind him.
Gratz, now you can raid Blackwing Lair!
To enter Blackwing Lair just go and kill that Scarshield Quartermaster again and touch the orb that he is guarding, that will teleport you into Blackwing Lair.