@KaizokuOLuffy:
Because he wants to be more powerful and can't use Haki?
That can be accurate, actually, if Blackbeard was truly that audaciously reckless when he boarded Whitebeard's ship, hoping to have a higher chance of finding the fruit that he was interested in; however, I believe he had a more elaborate, preset idea in mind when he took the first step into his large-scale scheme, which is why I question his purpose in desiring the Yami Yami no Mi, out of all of the other fruits….Then again, he was probably just that simple-minded, and merely wanted it out of personal conviction, which isn't too far-fetched, I suppose...still, though, I assume that the likeliness of such a fact is somewhat low.
[[b]EDIT: You do make a considerable point, however, concerning Teach's ability to use Haki. I presumed that Blackbeard should've been able to use Haki as well, but he can also be under the same case as Ace was, where Haki was simply never his strong point…]
@isrnick:
Haki doesn't make people unable to use their DF power… It just allows the CoA Haki user to hit a with a DF power directly as if the person didn't have its power, which isn't the same as to make the person with a DF power unable to use his DF power.
While that is indeed true, isn't the purpose, (which is to become capable of making contact with them), still the same, nevertheless? Unless removing the abilities of Devil's Fruit users give Teach an advantage in which CoA isn't able to equally provide…
Still, though, you're right; technically, Haki isn't the same thing as the power of "absorbing" DF users' powers, but it was my fault since I had stated the question [that's being referred to] improperly, as I was primarily referring to the actual point of both abilities, rather than their functions.