I don't think Ryuma is on a whole other Level than Oden
If Roger turned Supreme Blade into Black Blade, it remains Supreme
And if He turn Worst Meito into a Black Blade, it becomes Supreme Blade as well
There isn't Higher than "Supreme" Grade because that's Max Level there is
If Strongest Haki Fighter ever used Hardening, it won't be better than a Supreme Blade
Another Example is Zoro, when He covers his Blades in Armament, all of his Three-Swords become of Same Durability
And this is why when Zoro turns all his Blades Black, They will all become Supreme Grade
Ryuma turned Shusui into Black Blade & yet it remained "Great Grade", meaning his Haki Level wasn't worthy of Supreme-Grade.
This is why Luffy was compared to Ryuma for matching Kaidou who was compared to Oden, and same for Zoro
Luffy, Zoro, Oden, Kaidou & Ryuma are All Fighters with "Great" Haki
But Mihawk, Roger & WB are Fighters with "Supreme" Level of Haki
Current Zoro can give Ryuma one hell of a Fight
But He can't do the same with Mihawk
Please be Patient, We are in Final Saga & Oda only revealed 3 Supreme Blade Wielders & not much from Great Grade as well
Forging Black Blade requires "Infinite" Haki, but it doesn't mean it's Strongest Quality-Wise