What do you mean!? How is that relevant
It doesn't have Haki on it. Monet would've died if shusui has lingering Haki.
Okay stay with me here.
This is how I think black blades work.
You know how heat is used to forge swords? When you apply lots of heat and impact on a sword it becomes stronger.
That's how I think Black blades work. You apply Haki on a sword and impact(from battles) on a sword it becomes stronger.
But as you've noticed SWORDS AREN'T HOT. After the Forging of the sword in heat is complete, the sword is permanently stronger BUT IT HAD NO HEAT IN IT. The heat dissipates as usual. But the sword is forever permanently stronger.
That's what black blades are with Haki. The Haki is used to permanently change the swords' color and make it stronger, but then it disappears like heat disappears. Leaving a stronger sword WITH NO HAKI.
Shusui would've cut monet in half if it had Haki in it. It doesnt. Haki can be used to make black blades WITHOUT the sword permanently having Haki in it the same way heat is used to make swords but the swords don't just permanently have heat in them