Blackbeard is really strange in that he was a member of whitebeard'a crew for like 30 years, yet he never obtained a bounty. Furthermore, before obtaining any devil fruit powers, he was able to kill Thatch, one of the division commanders (although that was probably a matter of stabbing him in the back rather than a straight up fight), and, even more impressive, scar shanks's face. From his fights, he seems rather vulnerable to attack, and not all that skilled at combat, just incredibly tanky.
Then again we've only seen him fight Ace, Whitebeard, and Magellan, all of whom are monsters.
It's hard to get a read on him.
He might have gotten Shank's face through a sneak attack during a larger fight. It would certainly fit with his personality.