if turning a blade black was simply fulfilling the will of the sword , is it really in anyway impressive?
I would like to think its a lot more complicated than that , maybe it has to do with one's armament mastery
but personally I have always seen it as a powerup reserved for future arcs since the only swordsman who currently has one is the strongest one and we are supposed to believe zoro is still significant ways off from that.
if zoro were to leave this arc with a black blade for hypothetically killing kaido after luffy defeated him
it would honestly justify the argument that forging a black blades has little correlation with one's strength as characters like shanks, oden, roger , rayleigh all in their primes are no doubt stronger than zoro who just forged one.