Rocks D. Xebec, better known as the Rocks, was the captain of the Rocks Pirates. He was prominent 40 years ago, before Pirate King Gol D. Roger came to power.
Although Rocks’ individual strength and abilities are unknown, he was reputed to preside over an extremely powerful pirate crew with opposing personalities and no camaraderie, indicating that he was an extremely powerful and charismatic individual and possibly the person strongest of its time. Many of these people went on to make a name for themselves; Whitebeard, Big Mom, and Kaido even became Emperors of the New World.
Oda made it clear how much Blackbeard is inspired by Rocks D. Xebec. Blackbeard gave his ship his name and obviously idolizes it – Xebec’s Saber.
What is another important point of the character of Blackbeard? He likes a healthy dose of devil fruit. He will fly and find his favorite fruits and use them.
He also seems to know about rocks. If Blackbeard is really like that, it makes sense that he’s trying to keep up with Rocks. If Rocks was a haki supremacist like Kaidoso I don’t think Blackbeard wouldn’t have tried to get two devil fruits for himself and keep his haki pretty weak (in Impel Down he was weak as hell if he really valued the haki, I think he would have learned something by staying with Whitebeard).
Besides, I think that’s also why Kaido believes that haki is supreme. He was present at the God Valey and there he saw his Captain Rocks, someone who believed that the fruits of the devil had the power to rule the world, shot by whom? Garp and Roger, who only used haki.
Kaido learned that his captain’s philosophy was not correct and continued to improve his haki to the highest level.
I don’t believe Blackbeard will have super strong or advanced haki. The fact that Luffy won’t be able to use his Devil Fruit against Yami Yami no Mi means the fight will be pretty fair, but obviously, as Oda told us before, haki reigns supreme and that’s why Luffy will win.