Okay, I wanted to post this in the original theroy blog but I can not.
Alright.... since blacbeard was able to gain the powers of two Devil fruits it seems likely that he can not actually have to Devil Fruits in his body.Else his body would expoled. Yet, it has been comformed in the canon that Blackbeard's Yami Yami no Mi is unique amonst the logia types. instead of turning the user into the element to protect him, he instead absorbs the attacks he is hit with, and he was able to absorb an entire town into his darkness and attack Ace with it's remains. So, it is quite possible that he casn only use the abilities of WhiteBeard's Gura Gura no Mi but does not actually posses said fruit and that the Gura Gura no Mi is still in circulation. Thus that would explain Blackbeard being able to use the fruit's powers without dying due to eating 2 fruits.