didn't they say that having more than one devil fruit will make you explode?
I can understand the power stealing, but it requires some explanation about dual fruiting.
anyway.
loved that marine, who's soiling himself because he's so afraid of earthquakes.
also, WB was 72? I thought he was 50 something, that makes him even more amazing. we really need some more flashbacks about him, maybe after all this is over, maybe we'll start hearing about islands in NW that WB visited, maybe the merman island will give some more info on the subject...

back to Blackbeard.
here's my take on the issue.
Devil fruits only grow inside a dead man's body. which explains how there's only one person of each power, and yet the fruits can be identified from sight.
Black beard has a twin inside him, (like the guy in basilik), which BB feeds the evil fruit to, and when he wants to switch backup powers, he sucks the fruit out of his brother, and then feeds him another fruit.

I'm positive that the secret to the power swap isn't just the blackwhole, since that guy commented about BB's atypical body, and furthermore, most devil fruits abilities are recognized, and if someone in the past had the ability to steal powers, it would have been made public.

or something. anyway, Blackbeard hides the process because it's his weakness.