@Mr.:
The yami yami no mi is an alternate for CoA, but is doesn't account for mantra and CoC. Although I doubt that lackbeard posseses CoC if he did know how to use haki and since he's always being caught off guard he obviously doesn't posses mantra. He apparently knows about Haki because WB and his top commanders had it but I doubt he knows how to use it.
Yeah. All in all, I think Blackbeard is just a talentless, untrained oaf, but through several prudent moves and careful planning, he's become the most powerful person in the world.
And Luffy, the opposite of talentless and untrained, will defeat him because of this. If it's "arrogance" and "thoughtlessness" that nearly killed him at Marineford, then it'll certainly be "modesty" and "thoughtfulness" that do him in at Raftel.