I think Blackbeard is a very interesting character, his appearance is a stark contrast to both his power, and personality. he's the very example of a scheming villain (although I can't say he is very evil, the WG is vastly more sinister than any pirate could be.), but he happens to be housed in a brutish thug's body.
Odd theory of how he was able to consume more than one devil fruit: Since the majority of his mass is his gut, I think being able to eat more than one devil fruit has something to do with your body's ability to digest more than one, since they taste horrible and may have adverse effects on your health if mixed with another. Maybe eating more than one is like poison, so building up an impossibly resistant digestive system would allow you to consume more than one fruit.
Also, since his Fruit controls gravity, and since not even light can escape that, perhaps he was able to consume the 'power' (since Kuma is able to 'reflect' concepts like pain, which have no physical form) of whitebeard's devil fruit through that same gravity, thus not actually having to eat the fruit. This would also explain the reason for the large tarp that had to be thrown over, assuming he needs a larger shadow to produce a stronger force of gravity. I deduced that from seeing most all of his darkness protruding from his back (again assuming a light source would be shining towards him).
Honestly I prefer the Cerberus/Chimera theory, but I just wanted to throw that out there.