I'd like to bring up one thing that could be a misconception when it comes to black blades among the fandom
When a Black blade is "forged", it does not increase in rank in terms of the meito system, so for example a great grade blade, becoming a black blade, does not make it a "supreme grade" blade. That is just how we have chosen to interpret this information
That system just indicates the quality of the blade when it was made, that actual rank does not change, when it comes to the grading system.
Japanese readers, seem to understand that when it talks about it elevating in rank (after becoming black), it's actually completely separate from that rank system, forging it into a black blade is an echelon of it's own
This is talking about another rank entirely, and the blade being elevated in status separate from that, it's literally it's own league of status
This is further backed up by more recent statements we have, which say Black blades are the most elite blades in existence, this literally goes beyond just the grading system :
To give an example, and I'm gonna use a DBZ example here as most people would understand that
You have your super saiyan line, ssj1, 2, 3 and 4, and the saiyan is using their own (mortal) ki in those power ups
And then you have
God ki, Which is an entirely separate type of ki, and when you apply that to super saiyan
You get SSJ Blue
If base form is a low rank grade, ssj1 is slighter higher etc, ssj3 is great grade and ssj4 is supreme grade
and then you add God Ki to that transformation, that is what making a black blade is
So even if you take shusui, that was originally great grade, but you're applying "god ki" to it when u make it a black blade, and it's in an echelon of it's own, in terms of durability black blades are meant to be unparalleled.
You might think this potentially cheapens supreme grade blades or something, but that's not true, a black blade has to be earned, just like a swordsman has to bend a high grade sword to it's will:
being forged through your own history of battle
That's why he then asks if zoro is greedy and just thinks about his base desires, using someone elses black blade, because he didnt forge it
zoro is later told that he can make enma a black blade potentially.