This theory will explore reasons we can find from the story that could explain why Cursed Swords exist.
The first clue we’ll start with is that black blades are made from a swordman’s Haki. That’s useful information but it doesn’t close this case. Because Black Blades aren’t said to be Cursed as far as we know.
Enma and the Kitetsu aren’t black yet they are cursed. However Haki isn’t only determined by that black color. Hyogoro described the flow of Ryou as being invisible.
So it could be that, but I’m more inclined to think it is Conqueror’s Haki. Before I explain why I have to go into who’s Haki is being infused into the blade.
While Black Blades are forged by a swordsman’s Armament Haki, I believe Cursed Blades are forged by the swordsmith’s Conqueror’s Haki. In fact it’s not a curse, but rather a test worthiness.
First it shouldn’t be to shocking that the curse is related to Haki. Haki can translate to spirit or as a willpower and many other similar words.
When Zoro first used Shusui he commented on how heavy it is and how he’d need to get used to wielding a blade of its material. Was this Ryuma’s will wanting a swordsman skilled enough to wield such a blade?
It does match up with what a swordsman would expect of another who wields its blade. Most swordsmen do not care about the intent of the one using the blade, Tashigi being a rare exception. However The swordsmith cares more deeply for their weapons more than anyone else. They want to know that its wielder can honor the sword. When Zoro proved himself to Sandai Kitetsu it wasn’t his swordsmanship(like Shusui) being put to the test, but rather his conviction(Haki = Spirit).
My last piece of evidence is what we learned from a swordsmith. Hitetsu tells Zoro that he is compatible with Enma due to his possible connection to it’s swordsmith.
This shows that the swordsmith does indeed affect the performance of the sword. So that just about covers why I believe Cursed Swords exist, in the next theory I put together I will go over what I believe the true potential of cursed swords power.
Here’s the video version of the theory:
*Theory by EliDZ