This is also why his body is so weird. He is still a human, but because of his Devil Fruit, his body changed. Most Devil Fruit users are affected by the fruit they consume, because they modify their Lineage Factor (DNA). Luffy’s body can stretch constantly, Marco can use his flames even in human form, and Chopper retains his human intelligence no matter which form he’s in. Because his body composition is different than that of a normal person, it is possible this might be the sole reason he can sustain using more Devil Fruits at the same time. One devil fruit for each of his hearts.
However, despite giving Blackbeard a huge advantage in terms of Devil Fruit usage, his Octopus devil fruit is a burden for his physical abilities. Since his nervous system is much more spread out throughout his body, it makes him more susceptible to pain, something clearly seen every time he gets hurts. Another drawback is that his body is much more sloppy, just like an octopus, and unable to move very fast. Even his stamina is bad, as he gets tired fast and is seen to be out of breath a number of times. Remember, he is 38 pre-timeskip, and is younger than Doflamingo. Just like an octopus, he may be unable to sleep very often, and Oda may even take this a step further and make him completely unable to sleep. In the image below, you can see that, despite the punch of Luffy not dealing much damage, Blackbeard is in agony, and is even a bit tired.
Teach’s fight strategy is also very similar to that of an octopus. He relies a lot on his intelligence, avoiding combat as much as possible, and even running away when the situation is not good. He has camouflaged himself as an innocent crewmate on Whitebeard’s ship for 20 years, and even did this while being a Warlord, hiding his true intentions from everyone.
There’s some other things that lead into this. The Jolly Roger of the Blackbeard Pirates features 3 skulls (reference to 3 hearts leads to what Luffy and Zoro said at Jaya) and 8 bones (tentacles). It’s also interesting how his other 2 Devil Fruits play into the theme of an octopus/Kraken. Krakens could supposedly create whirpools, and change the sea surface when they appeared (Gura Gura no Mi). The Yami Yami no Mi could clearly represent the ink an octopus produces.
In regards to the Tako Tako no Mi, Model: Kraken, like I said, the existence of a fish fruit increases the possibility of an octopus fruit being real. Octopuses can survive on land for a limited amount of time, so it would not be a problem in that sense. Regarding its special ability, specific to a Mythical Zoan, you have all the abilities of an octopus: regeneration, limbs still attacking after being cut, fitting into small places, climbing stuff with his suckers, and camouflaging, while at the same time being enormous, and using the other 2 devil fruit with its arms.
*Theory by grramramram