Before I start check out @Admiral Ryokugyu theory about BlackBeard https://opforum.net/threads/blackbeards-secret.83499/ In a way it plays hand in hand. I don't have much since we haven't seen it happen except I think once. If you remember back during Punk Hazard, Ceasar was able to give Smily the Sara Sara no mi, model: Axolotl. We see towards the end, when Smiley died that his devil fruit went into the nearest apple which happened to be right below him. We also know Blackbeard an his crew have been traveling the New World searching for and taking Devil Fruits (geez there are a lot more than we thought). On Dressrosa we seen Jesus Burgess try to take the gomu gomu no mi from Luffy. From seeing Smily an the way his devil fruit did, I believe the secret to how they BB pirates attain devil fruits is that the "soul" of the fruit come out an goes obviously to the nearest fruit. But that the literal soul becomes tangible and goes to said fruit. However, I also believe you do not need the actual fruit to gain the powers. I couldn't imagine the BB pirates always attacking someone with an apple or orange on hand. If I also remember right it has to go to the nearest fruit of the same kind it was. For example Kakus fruit was a banana and Buggys was a pineapple. They'll always be that. I believe they can literally eat the soul of the fruit an gain it. We know their are inanimate objects, animals & live experiments who have gained devil fruit powers However, I believe that the soul is invisible. By surrounding it with darkness it becomes visible and tangible. This would be why Blackbeard was covered with a black shroud during Marineford. He was able to see it and either had a fruit of the same kind or he was able to grab it and eat it. I believe they do something similar to everyone who they've taken fruits from. They dont need many sheets as big as Whitebeard so it's much easier to do if they're by themselves. As I said I do not have much proof besides that little bit and it's just a theory.