The answer is very simple. A sword or blade grade is just the quality of the weapon or how well the weapon was created.
When a blade is turned into a Black blade you don't improve the quality of the weapon. You just increase the strength of the blade. You make the blade so strong it can't be broken or chipped but the quality of the blade is the same.
When a blade is turned into a Black blade you don't improve the quality of the weapon. You just increase the strength of the blade. You make the blade so strong it can't be broken or chipped but the quality of the blade is the same.