The only thing we really know about Blackbeard from before he ate the Dark fruit is that he cut Shanks and killed Thatch. The problem is that we don't know how these fights went down. The fight with Shanks was over 12 years ago, so we don't know the extent of Shanks' power at that time. After reading wayzz's comment, I reread the conversation between Whitebeard and Shanks, and it gives a pretty good idea of Blackbeard's power. If any, you should read chapter 434 again, to get a grasp of his power.
It was mentioned by Hawkeye and later by Whitebeard that, Shanks and Hawkeye used to fight many times. Despite fighting many times, there was never a clear winner, so we can deduce that Shanks' strength must have been similar to that of Hawkeye's. Now getting back to the fight between Blackbeard and Shanks, Shanks mentioned that he wasn't being careless and despite that, BB could give him the scar on his left eye. He mentions that
Shanks: Blackbeard was waiting patiently for an opportunity... He didn't take the position of captain, nor did he become renown (by taking the position of division commander). He hid himself behind a large shadow called Whitebeard!
So being in the Whitebeard Pirates for many years, being able to scar Shanks in a real fight, being able to take down the fourth (Thatch) and second (Ace) division commander of the Whitebeard Pirates makes him a worthy opponent. Don't forget that two of the Yonkou got nervous about Ace fighting him. Both Whitebeard and Shanks wanted to stop Ace from pursuing him, because they felt like this was not going to end well, and it didn't. So it is safe to say that he didn't become the new Yonkou based just on his Devil Fruit powers alone.
I think Blackbeard's seemingly cowardly appearance might have something to do with his Dark fruit. The Dark fruit increases the amount of pain received by Blackbeard. So he might not be scared of losing the fight, but he might be scared of the pain that follows a single blow. His Dark fruit absorbs the punch even more than a regular person would, making it hurt much more. It hasn't been confirmed, but this might be the reason why he seems scared. Unless if you were talking about the Marineford Arc. At that time, I think he was honestly worried about losing, or at least getting his crew hurt or captured, because he would have never been able to fight both the Marine Admirals and one of the Yonkou at full strength. At that time, he chose wisely to retreat. This would give him more time to really master his newly absorbed Devil Fruit powers.