@Power:
Ok - Out there theory.
From what Marco said, that being that his body is abnormal rather than saying 'that fruit…' or something, while it may seem a far reach I'm going to assume BB somehow is able to absorb two fruits, even if he didn't have the blackness thing. A stretch, but if that's the case...
What if the process in how a devil fruit is reborn involves darkness. As in, the tree grows through long years in the darkness, or when it's pitch black the demons exit the body of the dead guy or something. Then maybe the blanket and the darkness fruit would have been dark enough to speed it up and BB somehow managed to immediately snatch it's power (i.e. eat fruit or absorb the 'devil fruit's spirit, etc...). It just seems too showy to show up in the middle of a frantic war with a blanket just only to make a grand entrance.
True but even Marco seems unsure how Blackbeard is able to consume 2 Devil Fruit.
Also, for those who think Blackbeard's body is the reason he can consume at least 2 Devil Fruits, why the need to wait years to obtain the Yami Yami no mi?
I have not seen this discussed by why does Blackbeard want to hide what he did to Whitebeard's body? Teach is a showman and braggart, so why hide this technique?
Before reading the chapter on Mangastream, I assumed when people said Black Cloak, they were mistaking it for Teach's vortex but the guy really did hide what he did with a cloak. Of all the things that happened, this is the most interesting action taken by Teach.
Just maybe he hid this technique because others could mimic it?