@AdmiralYonkouMt.Bandit:
Technically, we don't know if BB used his DF to absorb WB's DF. Oda seemed to be directing us towards BB's unique physique with Marco's comment.
It could've been as easy as BB plunging his hand in to WB's chest and ripping the DF out and then eating it or something more complex. Still don't know why either needed to be hidden from us or the spectators in the war unless how and where a DF shows up after it's previous user is a secret. If BB's yami yami no mi was capable of absorbing DFs I figured he'd brag about that too.
This may sound stupid but what if he wasn't hiding it? Maybe Teach needed a dark place to talk to the devil inside WB? Kinda like convincing him or something? (the devil of the DF of course).
It sound stupid but just wanted to throw that in.