You get Blackbeard by doing this. First, complete On Stranger Tides. Then, go to the little area on the left. Take a barrel, and roll it into the thingy. Once the bridge is down, go into Tia Dalma's hut. I forget which level he's on, but once you find him, fight him, but don't kill him. Otherwise, you've lost him. Enjoy Blackbeard!!!
P.s. You also have to be rich in order to get him.