The way Blackbeard absorbs ones Devils fruit powers is through his own Devils fruit.
Blackbeards Yami Yami no mi is unique even for Logia, there are couple of traits that make it unique. (Not gonna go into them).
The Yami Yami no mi's main power is to absorb and devour anything.
In my theory, the absorption and devouring even includes DF powers.
As we have seen, when BB uses his DF when fighting other DFs, when he touches them with his darkness, they lose their DF power. My theory is that the darkness tries to pull the DF power and devour the DF power(Much like Pains soul absorption in Naruto), but since the user of the DF is still alive, BB cannot completely devour it, only pull it in, and once he loses contact with the opponent, the powers return into the opponent.
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In my opinion, when BB is fighting a DF user, and nullifies their power when hes touching them, his darkness powers are actually trying to absorb the opponents DF powers, but since the opponent is still alive, the powers are still strongly attached to the persons life force, soul, body, whatever. Once the opponent is dead, there isn't anything stopping BBs darkness from absorbing and devouring the opponents DF powers.
