So to start off, as we all know, the Gorosei have a devil fruit that they fear who’s name they’ve changed to hide it from public knowledge.
A lot of theories assume that legendary fruit is Luffy’s Gomu Gomu no Mi, but honestly I don’t see it (due to personal bias of wanting our rubber boy to stay rubbery).
So, as we all know, the Dark-Dark fruit is a Logia fruit which Blackbeard ate after killing Tatch and betraying Whitebeard.
So, what’s so special about it? First, let’s go over the abilities we’ve seen so far:
1. Place pools of darkness which can absorb anything, be it alive or not.
2. The darkness seems to also have a tangible form, being able to be used as a projectile or grab opponents.
And now, the abilities that stick out to me the most:
3. Draw people into said pools like a blackhole, which can be used in even small volumes as demostrated by Blackbeard grabbing Luffy and Ace
4. Nullify other devil fruit abilities.
Although not as relevant, I also find it weird that Blackbeard’s devil fruit seems to have no drawbacks when it comes to the sun/light in general, as even Moira’s devil fruit, the shadow-shadow fruit, still had something related to it. Also, the explaination for it sucking it objects is weird, as Blackbeard says:
Which, as we all know, is not actually true. As I was rereading the Ace VS Blackbeard fight, Blackbeard said something that stuck-out to me:
As Blackbeard states, he doesn’t “nullify” Devil Fruit abilities, he “absorbs” them using darkness.
So, where am I going with this? Simple: the fabled fruit whose name was changed by the Five Elders due to fear of its powers is the Dark-Dark fruit and it’s true name is actually “Void-Void Fruit”. If Blackbeard’s true devil fruit is actually the Void-Void fruit, then it would explain his gravity related abilities a lot better, as he would be making a “void” that absorbs and crushes gravity around it. It would also explain its unexplained ability to nullify other devil fruit abilities, as he would be making them “void”, if you get what I mean.
I believe Blackbeard’s devil fruit being that legendary fruit makes a lot more sense, as we have already seen it’s terrifying destructive power:
As well as how easy it is for it to take out multiple people at once as well:
Now, imagine just the black hole ability alone but now in a Awakened state. Although Awakenings as a whole are still a mystery to us, as we still aren’t sure how each devil fruit is affected by one, I’m gonna have to use the theory that an Awakened Logia is able to turn objects into their respective ability and greatly increase their abilities range.
Imagine how powerful the awakened “Void-Void fruit” would be like that. It can already swallow an entire small town as we’ve seen, so in an awakened state, it’s entirely plausible that it could swallow entire islands and take down armies alone. A devil fruit like that poses an immense threat not only to the Government but the very world itself. It could also possibly make Haki “void” but that’s just speculation.
Whether or not Blackbeard knows of this possible fact is unknown but I wouldn’t put it past him somehow learning about it in yet-to-be-seen backstory/flashback. But if he did know that, why would Blackbeard also eat Whitebeard’s Tremor-Tremor fruit, since it’s already known to be capable of destroying the world? My theory is combat versatility for Blackbeard, as, as far as we’ve seen, his devil fruit does not fare well in close range combat and can easily be worked around to combat it
So, a powerful devil fruit like that which is much more effective in close range combat would come in handy to make you seemingly invincible in almost every combat scenario aside from ones against an elite few.
If my theory is correct, I think it would elevate Blackbeard as an endgame opponent a lot more since Kaido holds the title for strongest creature alive while Blackbeard is as dangerous as he already is. It would also make Luffy feel like the underdog he always is, as he would need to defeat an opponent like that while being a simple rubber man.