A black blade is a weapon of a classical swordsman. People like Mihawk who can cut nothing and everything.
Roger, Rayleigh, WB, and even Shanks aren't classical swordsmen. Bar WB, who uses a glaive, they are swordsmen. There is no mistaking that. However they don't follow the same philosophy of swordsmanship as Zoro and Mihawk. They don't care about using their blades to cut.
Rather, as we were directly shown with Shanks and Roger's Kamusari, they use internal destruction haki with their swordsmanship to send out shockwave slashes.
Is it still swordsmanship? Yes. However such a style of swordsmanship, a style that doesn't actually cut the opponent, does not require an ultra hard blade as is required of someone such as Mihawk, who wishes to cut everything(and nothing).
Roger, Rayleigh, WB, and even Shanks aren't classical swordsmen. Bar WB, who uses a glaive, they are swordsmen. There is no mistaking that. However they don't follow the same philosophy of swordsmanship as Zoro and Mihawk. They don't care about using their blades to cut.
Rather, as we were directly shown with Shanks and Roger's Kamusari, they use internal destruction haki with their swordsmanship to send out shockwave slashes.
Is it still swordsmanship? Yes. However such a style of swordsmanship, a style that doesn't actually cut the opponent, does not require an ultra hard blade as is required of someone such as Mihawk, who wishes to cut everything(and nothing).