We can safely assume that a Black blade doesn't mean that That Blade is always covered in Haki, otherwise Monet would've died immediately.
There's no plot way how Oda can write his way around that even if he tried.
I think it's purely about quality. Mihawk takes extreme pride in his blade, and every damage is a shame/attack for his own ego, so I guess once you turned your blade black your Katana will never be broken again.
A Black blade = highest durability for a swordsman