@DHL:
I don't think it was just the fruit.
And honestly i do think he has Haki. He would be the biggest hypocrite because he would be dependant on his fruit.
Blackbeards plan was to;
Capture Ace,
Hopefully create an entire war (as he knows whitebeard won't sit back and watch)
Have Whitebeard weakened.
Kill him and then take his Devil fruit.
I agree. Blackbeard must be quite strong without his devil fruit if he was able to hurt Shanks.
But I disagree on Blackbeard's plan being to capture Ace. Blackbeard is not the planning sort of guy.
He had one goal for which he outlined a few simple steps.
He wanted to get strong people from Impel Down
To go to impel down, he must have connections with government, i.e. he should be a Shichibukai.
To become shichibukai, he would have to do something big.
And that's it. When Luffy defeated Crocodile, Blackbeard thought defeating Luffy would be good enough for getting in limelight.
Ace was never his concern from the start. It was Ace who butted in his "plan" of attacking Luffy.
There is no way he could have planned that ahead. He just thought - "All right, Ace if notorious too. Capturing him instead of Luffy would good enough."
He didn't plan to weaken and defeat Whitebeard from the start. But when he saw Whitebeard fighting for Ace, the opportunity came before him and he took it.