Several theories speculate about Blackbeard being Rocks D. Xebec’s son. In the meantime, some people are bringing up that Blackbeard might just be a Rocks fanboy in the same vein as Luffy idolizes Shanks.
As we know, Rocks’ full name is Rocks D. Xebec and Blackbeard’s ship is called the Saber of Xebec. The Rocks Pirates had their home base at Beehive Island, the same island Blackbeard is at now. It could be Oda drawing parallels making Teach Luffy’s “Rocks”.
What if Rocks D. Xebec was the previous owner of the Yami Yami no Mi and Blackbeard acquired it knowing everything?
That’s why Blackbeard claimed that the Yami Yami no Mi devil fruit was calling him in some way. Teach is the inheritor of Xebec’s will.
That’s also part of why Rocks D. Xebec had the upper hand over such strong people like Whitebeard, Big Mom, Kaido and Shiki.
Any big figure in the world that wants to rule over the world in the same way as Xebec needs the Yami Yami no Mi as it brings Darkness to the world.
I also think that Blackbeard took Whitebeard’s fruit not only because it was powerful, but also because it helped him get just a little closer to his image of Rocks D. Xebec (connecting yet another part of the Rocks Pirates to him). The more elements, the better.