@freedom:
If what you're saying is true, then someone could catch BB off-guard, stab him in the head, and he'd be finished.
Blackbeard clearly lists blades as something his power of darkness can absorb. "Darkness sucks in everything. Bullets, blades, blows, blazes and lightning blasts!!!" How it all works is a little hard to understand, but we know he feels pain. I originally took it as "any pain that is enough to physically damage me will get absorbed". Basically, a stab would be absorbed, but he would still get stabbed, which hurts like hell. He can still be punched, kicked, etc. But anything greater than normal, like a gunshot, stab wound, burn, etc, would all get absorbed. It could mean that everything hurts more than it normally would, like he is absorbing the maximum amount of pain possible or something, but that doesn't make much sense to me. Vanessa explained it well once, but I don't recall her explanation.
@freedom:
I've always been confused a bit by BB's "special" logia circumstance.
I always figured it worked as its power implies. Logias can create and control their ability, but more importantly they can become their ability. If you think about it that way, Blackbeard fits the criteria. He can create and control black holes (ie: gravity, etc), but he also becomes a black hole. Since his ability is infinite gravity, rather than shrugging off attacks like a normal logia, attacks are forcefully pulled into him. It sort of makes sense that he cannot disperse his body the same way other logias can as well, since he is gravity. Then again, Oda could have a different explanation later on down the road. This is just my stab in the dark theory.