After seeing and reading the manga i really think that Blackbeard has multiple personalities, 3 personalities to be specific.
He was able to stop an enraged Whitebeard's quake attack with his own physical strength and with the help of his Kurouzu technique.
He's overconfident in his abilities and inclined to underestimate opponents, the Yami Yami no Mi does not give him access to an elemental state where he can disperse, reform and regenerate, his gravity powers has the perpetual consequence that things such as his opponents attacks are attracted to him, which increases the power of the attacks and consequently the pain he takes from them.