Interesring and it confirms few things
- A sword can increase in rank by becoming a black blade
- Emma is a stronger sword than Shusui naturally, but currently it's in the same lvl as Shusui because it's not a black blade
- Oden didn't manage to turn Emma black, so Ryuma had better Ryu mastery had Oden
Though it makes me question what was Shusui's original grade? Was it even lower than now? What happens to a supreme sword if it gets turn black. Will it be of an even higher grade?
So is the manga saying Shisui is a supreme grade since it is a black blade. This is the first time we have even received this information so now that question is, is Shisui current grade based on the fact it is a black blade. I would say no because this information is coming from a swordsmith (Hitetsu), who should have a better understanding of a weapon grade then a normal person. If someone doesn't know that it's reasonable for them to not rank the sword based on the fact it is a blade black, that's assuming they know how a black blade is created.
For example: If Shisui was a 21
O Wazamono grade swords when it was created and now it's a black blade. If the person using it doesn't know how black blades are created nor did they know a black blade increased a sword grade then there is no reason for them increase the sword grade.
What's even more interesting is Wado Ichimonji was created by the same person who created Enma. That means if Zoro turns that blade black it will also be a supreme grade blade.