I mentioned before that a lot of people seem to think that Blackbeard's Fruit is actually a Moon-God Fruit to match Luffy's Sun-God Fruit. I disagree with this. I DO think it's likely that it's a Mythical Zoan, though.
I don't think Blackbeard's Fruit will end up being the Moon-God (I'm not even sure if that's going to be a thing. A Moon-God wasn't in the list of the 4 Skypeia Gods, way back when). And I don't think it's the Earth or Stars, either.
I think it's basically the darkness BETWEEN the stars.
Just like how the opposite of love isn’t hate (it’s apathy), the opposite of the sun isn’t the moon (it’s the darkness). Blackbeard’s fruit is most likely not just darkness, but COSMIC DARKNESS. The Darkness that existed before time and the universe. I think Blackbeard’s fruit will end up being based on Amatsu-Mikaboshi from Japanese mythology.
https://occult-world.com/amatsu-mikaboshi/
At first, I thought MAYBE the Yami-Yami Fruit could, somehow, be the original Devil Fruit. The one that begat all the others, since it seems able to absorb other powers into it. Like how Amatsu-Mikaboshi was, in some myths, the primordial darkness that existed before light and life were created into the universe, and was then shattered into pieces to create the universe itself.
But, I think the truth might be linked to the Three Skulls on his flag. More on that in a bit.
I get the feeling that whoever or WHATEVER is the Will of Blackbeard’s fruit will likely end up taking him over and going out of control by the end of the story. He might end up being a giant black hole that everyone needs to team up to stop together. It MIGHT end up being Rocks...or maybe Rocks was being influenced by this, too.
Also, a lot of people have been speculating about what Blackbeard’s final power will be. Some people have been speculating that since he has a Logia Fruit (the darkness), and a Paramecia Fruit (Whitebeard’s tremors), that he’ll get a Zoan fruit. BUT, if the Yami-Yami Fruit is actually already a God-type Zoan, that means he’ll need a Logia.
And…I think it’ll end up being Akainu’s Magu-Magu Fruit ( the Lava Admiral that killed Ace). SOMEONE has to take out Akainu for killing Ace, but this story is already pretty packed. It’d be…economical for Blackbeard to “steal” Luffy’s revenge and get a power-up for it. And that way, Luffy can kill two birds with one stone by taking down Blackbeard and Akainu by proxy in one fight.
I’m not even 100% sure that Blackbeard’s Devil Fruit…is actually a Devil Fruit, honestly. Since it seems to work completely differently than every other one we’ve seen. Or maybe…this is the only TRUE Devil Fruit? Maybe this is the Fruit that gave all the other Fruits a “bad name” and got them labeled as “Devils”? A random thought, but it’s a possibility.
Maybe these "God" Fruits are containing the personalities of the people that created the Devil Fruits? As in, a person will BECOME a Devil Fruit if they MAKE any Devil Fruits, as a form of human sacrifice?
Anyway, onward to Blackbeard's body.
A lot of people are theorizing that Blackbeard actually has 3 hearts. Which is why he's able to possess multiple Devil Fruits. There's a comment by Macro about his body just being strange...in a vague way.
Maybe he has a parasitic twin, like Quaid in Total Recall. Maybe he's a clone of Rocks.
We really don't have enough information. But, not only does Blackbeard have a strange Devil Fruit, he also has a weird body.
And tnen there's the Three Skulls.
There's...something going on with the Three Skulls.
Blackbeard has them on his pirate flag. Oars from Thriller Bark and Little Oars the Third had 3 Skulls around their belts. In Film Red, there's a "Devil King" (Tot Musica) with 3 Skulls, which Oda says is thematically important to the main story. (I haven't seen the movie yet...)
So, Blackbeard is connected to Rocks...AND the Oni/Oar's race...AND the "nightmares" that Devil Fruits can bring to life.
What does that mean? I don't really know...
If we knew what the 3 Skulls represented, we'd probably figure out a LOT about the story....but it's really vague so far.
Maybe it represents the past, present, and future? It could be a visual way to show "Inherited Will".
Maybe it's about not letting go of the past? Of holding onto a grudge, rather literally.
Maybe it's about people coming together? A mass of people believing the same thing and making it into reality via the Devil Fruits? Maybe Blackbeard's Fruit was actually made by 3 people working together, which is why he can control multiple Devil Fruits? Maybe that's why he can't sleep...one of the three people in his head is always awake.
Blackbeard's version might not even mean the same thing as Oars' version. Blackbeard MIGHT have corrupted or twisted the "true" meaning.
And then there's the connection between Blackbeard's and Luffy's Fruit.
There's...something going on. From their first interaction Blackbeard and Luffy have been...two sides of the same coin, really. Luffy...DISLIKED FOOD around him...that's just WEIRD to me. Luffy loves ALL food. He seemed to just dislike it...just because Blackbeard liked it.
And Luffy seems like he should AGREE with Blackbeard's statement that "Dreams...NEVER DIE!" But, Luffy looks...horrified? Worried? Why?
So, maybe it's the Nika Fruit. Maybe the Sun God was the only one out of the 4 Skypeia Gods who disagreed with the others. Another possibility for the 3 Skulls.
Or...maybe they're twin Fruits. Two Fruits created at the same time by the same person...with conflicting Wills. A person who couldn't decide whether he wanted to save humanity...or destroy it.
It's possible the Dark God wants to continue the endless cycle of Inherited Will and basically live forever. And the Sun God wants to break the cycle and move past it. The difference between "Inherited a DREAM" and "Inheriting a GRUDGE".
TLDR? There's...A LOT we don't know, yet.