CoC infusion is basically like Kaioken. While CoA has different properties (hardening, barrier, internal destruction), CoC is just a reinforcement. That's why Luffy said that his attacks were still too shallow, only to infuse then infuse them with CoC and have more potency.
I guess Black Blade = high top tier level of CoA + perfect flow (since through flow, the swordsman becomes one with the blade, and the better the flow, the more capable of cutting is the swordsman), that coupled with a long history of fights. After all, it was said that Ryuma "forged" a black blade. And forging takes time, but it also takes craftsmanship (flow) and ability (Haki). So I guess top tier swordsmen like Roger at least lacked one of those. Since Roger fought a lot, this is off the table. I also doubt Roger's CoA was too weak, though since he was focused on CoC, it wouldn't be unrealistic for Ryokugyu to have better CoA.
I guess Black Blade = high top tier level of CoA + perfect flow (since through flow, the swordsman becomes one with the blade, and the better the flow, the more capable of cutting is the swordsman), that coupled with a long history of fights. After all, it was said that Ryuma "forged" a black blade. And forging takes time, but it also takes craftsmanship (flow) and ability (Haki). So I guess top tier swordsmen like Roger at least lacked one of those. Since Roger fought a lot, this is off the table. I also doubt Roger's CoA was too weak, though since he was focused on CoC, it wouldn't be unrealistic for Ryokugyu to have better CoA.