I believe that the dark dark fruit (aka Yami-Yami no Mi) will end up being a Zoan-type like the Gomu Gomu no Mi.
The first thing to note is how Blackbeard and Luffy are both mirrors, and opposites of each other. This has been since the very first encounter. They both order pie, one hates it, the other loves it, they both have similar ideas of piracy and dreams, but opposite ways of going about it, they both wanted to make the strongest pirate crew. I could go on, but you get the idea. Lots of similarities, yet fundamental differences.
I think this pattern will continue by Blackbeard also being revealed to have a special Zoan fruit (dark dark fruit) like Luffy, but the opposite kind. After all, Luffy’s devil fruit is one that represents the sun god, and what is Blackbeard’s? Darkness, the opposite of the sun. Well, actually, his fruit doesn’t really make much sense as darkness does it? Both in the name of attacks, and the way his powers work, a black hole is more accurate than darkness. (Also worth noting, the similarities and differences between the sun and a black hole. Objects in space, one gives light, the other absorbs light, black holes even come from the death of stars. Clearly, there is a relation there) so perhaps Blackbeard’s devil fruit is a Zoan-type of some kind of black hole god?
Also, one very important thing to consider, the fruit doesn’t even work like a Logia. The user can still be hurt when he’s supposed to be immune to normal attacks. From the very get-go, something has been strange about that fruit.
And one last thing to note. Given the parallels that Blackbeard and Luffy have, it’s not hard to imagine Blackbeard will be the final enemy Luffy will fight. And even with two fruits, I don’t see him having a chance if Luffy has the power of a god, not without Blackbeard also having the power to rival him. (Although maybe he’ll just eat a third fruit that’s a god type zone… who knows)
The fruit is also said to be the most evil, which A, seems kinda like it has a soul like a zoan fruit would, I mean, something can’t really be evil if it doesn’t have some level of agency. and B, adds another layer to it and gum gum being opposites. The Nika freed slaves and brought freedom and joy. While freedom is not exactly the opposite of evil, it’s very clearly a “good” fruit, the opposite of evil fruit. Also, during the fight between Blackbeard and Ace, Blackbeard told to Ace that his fruit chose him. More evidence that his fruit has a soul and is a god model.