Okay, off to make some sense of Blackbeard's power. Here is my rough draft understanding of blackbeard. Now, one of his hands enables him to use "darkness" that creates a blackhole that sucks in enemies, or to draw in an enemy and derives him of his fruit ability( right after taking in double-edge damage first as a trade-off to activate the ability).
Oddest thing is he was seen sucking in weak enemies without taking in any double-edge damage for a trade-off to suck in powers(his big entrance at Impel Down). Makes you think he could "pull" in anyone anytime.
His other hand, as shown in chapter 577, lets him "copy" a fruit ability and use it. I dont have any explanation for the cloaking between whitebeard and blackbeard. Some said it might be due to his abnormality in his body, his double or triple personalities, or something related to the symbols in the flag. It's just he haven't used his other hand to activate his other power until now.
In a sense, he only have two types of the same yami-yami power. He can suck in things, enemies, and even cancel their powers with one hand. Another that let him absorb anyone's fruit ability. Oh yeah, Blackbeard is ambidextrous when it comes to his powers so it doesn't matter if it has to be the right hand to absorb or suck in things.
Just a rough draft idea of blackbeard's power. Going with things that make sense or within the rules.