Its a black blade unmatched in its durability and it belonged to legendary figure someone called a Sword God ?
It's a black blade
That means it's in the most elite class. This should be enough.
That means it's > any supreme grade blade, that's not black
Why the fuck would it be so the make of the sword before anyone uses it, pre-determines the blades limit?
When the whole point is that only the swordsmen can forge it into a black blade, not any blacksmith, it's the pinnacle that only the wielder can achieve.
This is why I think the idea of a blade rising within the meito system upon being black is irrelevant, it can be argued that the statement made about enmas rank being raised doesn't have to necessarily mean it would become supreme grade, but black blade is another grade entirely.
Or that even if it was the case that shusui was below great than made into great grade, because it's been forged into a kokuto it possesses qualities that surpass the initial make of even the supreme grade blades, which is why black blades are the most elite.
Either way they are stated to be the most elite, not supreme grade blades.