Blackbeards body is different to other devil fruit powers, he is essentially a black hole, so he can absorb anything and pull it back out when needed. I believe he has Whitebeards power absorbed within his darkness power, rather then just sharing two powers. Which means he can probably contain unlimited devil fruit powers within his darkness power. What I am interested in knowing is what he did to Whitebeard under that sheet to get his power. Eat his heart??