@DaeJi:
No Haki does not cancel Devil Fruit powers. It can negate their defenses and COA can assist in defending against their attacks, but the fruit is still working at 100%. Being able to counter a power doesn't mean the the power is affected.
As for the Dark-Dark Fruit being better than Haki… every logia Devil Fruit power is better than Haki. Most Devil Fruits period are better than Haki.
By cancelling DF effects, it, by definition, cancels it's power. That doesn't mean the DF is completely useless while the haki user hits the DF user. The devil fruit is still in full effect for the environment and normal people. It just means the haki user is not affected by the opponent's DF.
@Muda:
No matter how fast your DF allows you to move, the Dark² fruit makes it impossible to avoid escaping it's pull. Blackbeard mentions this in his fight with Ace.
Granted, Ace isn't teleporting or super-speeding around the battlefield like Kizaru, Enel, or anyone with Soru might, but I would think that once BB has you locked on with Kurouzu, that's it; he's got you and you're getting pulled in, no matter what. Kurouzu pulls in DF powers, so it's a lock-on-and-pull-in the second he points his hand towards you…
Makes one wonder though: If you didn't have a Devil fruit, and Blackbeard used Kurouzu to suck you in, would you get sucked in or would it have no effect? Something about what BB said in the Banaro fight leads me to think that Kurouzu only works on peeps with DFs and not those without...
We haven't seen Blackbeard take on any epic fighters that don't use DFs, but I assume that his power will pull his opponent toward him. I wouldn't put it past BB to overestimate his ability, however. And even when he pulled in Whitebeard, he nearly got killed in melee.
Whitebeard's haki wasn't working correctly, but if he tried using his power on a Haki master, it may not work. But then again, Blackbeard is very strong; strong to the point that he could take on Shanks. But if someone's haki is stronger than the darkness fruit, Blackbeard's usual tactics won't work.