Black Blade is Permanent Hardening.
Anyone who knows Hardening, can learn to Harden their Blade,
But such Hardening/Black Blade will only stay Active as long as They keep their Haki Active & are in Contact with their Blade
Permanent Hardening means you make your Blade attached to a Haki Source that is "Always" Active & in Contact with Blade,
Something that Stays even after User's Death & can be Found "Anywhere", that's literally The World itself.
This is why Mihawk said "Know The World!" and "Take your Will, use it to Surpass My Blade".
I believe that Haki is like Shadows in Thriller Bark, They serve as another Version of you created from Absence of something.
Shadows are where Light doesn't reach, so i believe the reason Living Beings or anyone with a Will have Haki is because Haki is created from Absence of World's Will/Spirit
It's like People's Will act as a Barrier just like their Bodies act as one against Sunlight, creating Haki similar to how Shadows are created.
Obviously, Greater your Will, Greater your Resistance to World's Spirit, Greater your Haki. But no matter how Strong you get, it will always be Limited Potential, you must manually use that Haki & keep it Active whenever you wanna use any Technique
But just like there is DF Awakening, where you become One with your DF in Body & Mind, there should be a Haki Awakening, where you become one with the World (It's basically Sage Mode from Naruto and it's very common Concept in many other Fantasy Stories)
This Awakening allows these Haki Masters to have Infinite Reserves of Haki cuz They are using World's Energy, just like a DF User can use their Powers forever, cuz DFs are like Mini-Worlds and work as Infinite Energy Sources for their Particular Powers
By learning this, Zoro & Luffy won't have Limited Haki/Stamina anymore (For example, imagine if Roger's Haki was Limited, how can He match WB for many Days? cuz when both their Haki run out, WB would still have his DF, it's obvious that Best Haki Users don't suffer from such Weakness that Luffy especially still struggles with & Zoro also to some extent)
However, a Blade Blade is equivalent of making the World itself use Hardening on your Blade, hence why even after Death it remains so, and such Feat requires you to bend the World to your Will, not just be part of it, meaning it cannot be something that many can do, if multiple people are doing it then their Wills are going against each other's, hence why only One Person can sit on Throne for each Haki Technique
The World will only grant One Person "Permanent Hardening" or "Permanent Barrier" or "Permanent Internal Destruction" or "Permanent Future Sight" ... etc
So only One Blade Blade can be forged at a time, it represents World's Blade, it's like the World is a Ship & you're it's Main Swordsman/Vice Captain (Since it's Zoro's Part of Story)