It makes sense now, how Shanks lost to Blackbeard, because he faught Blackbeard before he ate the Yami Yami no Mi, back when he wasn't taking extra damage due to the effect of the devil fruit. Assuming Xebec is a haki monster like Oden, it's highly likely that Blackbeard inherited his body from Xebec, meaning he was likely STRONGER physically before he faught shanks.