Lego pirates of the caribbean: How to get Blackbeard's knife, the treasure of the dead?

You need to get through the fifth level of Chapter 4, and then meet Blackbeard in the innermost place on the left side of the port (where the customs are selected), and you can buy it if you defeat him. If you need to open the black skull door in the future, just use the character "Black Beard" and hold down the stunt key in front of the door (that is, the key of Jack's compass).