Blackbeard declared that the throne war against the strong one has already started and Moria should take sides with him.
But why Blackbeard wants Moria to join his crew? Doflamingo clearly stated that Moria is “too weak to be a Shichibukai”.
We all know that Blackbeard is a Specialist when it comes to Devil Fruits. He has done researchs on Devil Fruits for many years and I believe they are choosing Devil Fruits very strategically now. When Blackbeard met with Ace and invited him to join his crew, he said that he has already planned everything like he has already planned how he is going to be the Pirate King.
That certainly involves how he is going find the Laugh Tale. I believe Blackbeard currently doesn’t possess a Poneglyph or doesn’t know a way to decipher a Poneglyph.
Moria’s Devil Fruit can be very useful for this purpose since he can revive dead people through shadows. The personality of that person could be changed but their skills remain the same. Just like Ryuma, his skills remained the same as before and even though his memories were faint and fading he still remembered his days in Wano and how he slayed a Dragon.
I believe Blackbeard is planning to revive someone from the past who knows how to decipher a Poneglyph or where Laugh Tale is. But in order to revive someone, they need the dead body.