That Magellan poison situation was also the thing that riddled my mind for the longest time....
Ace flame has fully engulfed Blackbeard and burn his ass
But his Yami power removes/absorb/undone all that shit later on..........and you see what BB talk about his abilities
He probably couldn't heal himself
if damage is already done, but he probably could remove the abilities according to Yami power like he removed Ace flame from his body.
Until there is further explanation.
I see this as unexplainable, inconsistency power by Oda...........BB could undone Boas stone too if we're been honest about he's Yami power.
But it is what it is