Rocks D. Xebec, commonly known simply as Rocks, was the captain of the Rocks Pirates. He was prominent 40 years ago, before the Pirate King Gol D. Roger came into power.
Although Rocks’ individual strength and abilities are unknown, he was well-reputed for presiding over a crew of extremely powerful pirates with clashing personalities and a lack of camaraderie, indicating that he was an extremely powerful and charismatic individual and possibly the strongest person of his time. Many of these people would go on to make big names for themselves; Whitebeard, Big Mom, and Kaido even became Emperors of the New World.
One of the things that have been made pretty clear by Oda is how much Blackbeard is inspired by Rocks D. Xebec. Blackbeard named his ship after him and obviously idolizes him – Saber of Xebec.
What is another pretty important character point of Blackbeard? He loves a good strong batch of Devil Fruits. He will steal and find his favorite ones and make use of them.
He seems to know a lot about Rocks too. If Blackbeard is really like this then it only makes sense that he is trying to follow Rocks. If Rocks was a haki supremacist like Kaido, then I don’t think Blackbeard would not 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 haki I think he would have learnt something while he stayed with Whitebeard).
Also, on a side note, I think this is also why Kaido believes haki is supreme. He was present at God Valley and there he saw his captain Rocks, someone who believed Devil Fruits held the power to rule the world brought down by who? Garp and Roger, both of whom were haki users only.
Kaido got to know his captain’s philosophy was not correct and went on to enhance his haki to the highest levels.
I don’t believe Blackbeard will have a super strong or advanced haki. The fact that Luffy won’t be able to use his devil fruit against Yami Yami no Mi would mean the fair would be pretty fair, but obviously as Oda has already told us ‘haki reigns supreme’ and that is why Luffy will win.
Theory by Golden_TAG (https://www.reddit.com/user/Golden_TAG)