Black blades are the pinnacle of swordsmanship, so Mihawk should be > Xebec too
He can be, but Xebec doesn't fall under the current generation WSS umbrella, I still don't believe not forging a black blade inherently makes you weaker even as a swordsman. We don't know the key to it yet, but Xebec and Roger not having one leads me to believe it's a very specific requirement which isn't as simple as "this individual is stronger."
Xebec and Roger have clearly been presented as the pinnacle of pirates alongside WB. Even Oldbeard was presented as the strongest pirate as long as he wasn't sick.
Shanks has next to no wiggle room now though, and it's got nothing to do with black blades. It's the fact that we cannot say Shanks isn't a pure swordsman. Oda has to do some heavy lifting if he wants to undo this. No amount of feats will matter, Shanks can 1 shot Imu and BB tomorrow and it wouldn't even matter, you're actually cooked.