Darkness is a living essence of corruption, evil more potent than black magic it can absorb anything in it's way that opposes as threat by corrupting it's entire entity and turns it to the dark side in other words any grand power the darkness absorbs it can utilize that power.
what blackbeard might of done during marineford after whitebeard's death is he had the gura gura fruit under the cover and his living darkness absorbed the "devil" out of the gura gura fruit turning it into an ordinary fruit. but here's the main thing blackbeard didn't directly possess the gura gura ability if you remember that I referred to the darkness as a "living" teach controls the power of the living darkness while the darkness absorbed and obtained the gura gura fruit ability since the darkness is not physical person it couldn't have the devil fruit side effects meaning it can obtain all the devil fruit abilities there ever existed but it could also mean that the darkness might become fully sentiment and take control of it's user becoming a full on demonic being with ultimate power the only the light can overwhelm and here's the more frightening part it wasn't even in it's awakened stage.
there now stop making ridiculous and cringe worthy theories about teach's secret of obtaining two devil fruits
no unusual body structure
no zoan cerberus
and no fucking multiple people in one body
when you're speculating just think outside the box but don't over exaggerate it you can simply use the puzzle pieces to make your speculation but don't just create the missing pieces and claim them as a fact theories can be easily made with simple facts but because of the over exaggerated theories I want to stop and just let the manga confirm the facts but if you want to make your theory then you have to do research and use evidence and context to reinforce your hypothesis as a fact