Blackbeard took Whitebeard's devil fruit capable of earthquakes. No one knows how he did it other than his crew, but it should be extra impossible because already had a devil fruit. If a devil fruit user eats two, they explode.

The only person who seems to know what makes Blackbeard different is the first division commander of the Whitebeard pirates. They used to be nakama, after all. But he's hardly one to explain it.