People are missing the whole point of Black Blades,
Regardless of how They are Empowered/Forged, Oda didn't have to make the Change Permanant,
The Wielder can simply Activate that Power whenever needed, just like any other Power or Ability or Technique,
Forging a Black Blade means that you're leaving this Weapon for Future Generations, it's a Feat for Future, not for yourself,
Why would Ryuma give his Sword a Permanant Buff that remains after his Death if it's only intended to be wielded/used by him?
Ryuma was Protector of Wano, so He forged them a Weapon that can Protect them, and it's not just for it's Defensive Capabilities cuz that would mean only Wielder would be Safe, it's also for it's Offensive Power which can Cut what Non-Black Blades Cannot!
Mihawk forging a Black Blade means his Sword was also meant to be Left for Future Generations to Protect themselves, but He isn't seen Protecting any Place nor is He Loyal to any Kingdom or Island, so what does this mean? It means Yoru was Forged to Protect a Kingdom that No Longer Exists (or cannot be Found anymore), and without mentioning all the Hints that Mihawk is Ancient, I believe that He is from Ancient Kingdom & He forged Yoru to Protect it, just like Ryuma forged Shusui to Protect Wano,
In other words, I believe a Black Blade is Created to Protect from Something that is a Threat even to Ancient Kingdom!
Since Zoro is meant to Inherit that Role, and since I believe that Zoro is Created to be Luffy's Sword, while Sanji is Luffy's Shield,
Then imo this Threat is something that Only Zoro can Cut, while Only Sanji can Shield from, (And Sanji is Designed to handle Greatest of Fires)
In my opinion, it's "Forbidden Sun" seen in Elbaph Mural & mentioned in Harley,
(Sun God imo refers to One who Restored Sun, meaning Villain steals it & Use it, so while Luffy Fights this Villain, it's up to Zoro to Cut his Sun, and up to Sanji to Shield from it, )
In Conclusion, I believe Black Blade have Unmatched Killing Power (As Kouzaburo said, a Blade is Judged by it's Ability to Kill),
And Yoru being named "Night" further supports this, as Mihawk is Protector of World from Forbidden Sun, but Zoro will Surpass/Replace him,