So you think the Black Blade helps him kill Kaido?
Like Gyukimaru said - You will never be able to understand the true value of Shusui - means Zoro couldnt reap the benefits of the black blade because he wasnt capable/didnt know how since he never created on himself.
In my opinion, Kokuto just being extra hard was enough, invincible shield is a pretty dope thing but even without being Kokutos, Zoro's blades are already invincible shield, all 3 of them withstood an island buster. Hiryu Kaen was portrayed as enough to kill Kaido so we will see what he ends up getting or he gets nothing and simply tries again, perhaps allowing Enma to drain him completely.