Speculations I think Black Blades act like a Powerful Soul

[Impact]

1. Law demonstrated that a Devil Fruit ability affecting his body could be resisted by using a high output of Haki. This can be called active resistance.
- Law resisted Doc Q's gender-swapping virus after it had affected his body.

2. Kaido and Big Mom demonstrated that a Devil Fruit ability could be resisted before it affects their bodies because they have powerful Haki. This can be called passive resistance.
- Kaido and Big Mom resisted Law's teleportation before it affected their bodies.

I'm thinking if Law tried to teleport a Black Blade, he might discover it won't move.

However, if Law tried to move the user of the Black Blade, he might actually succeed.

That would mean Black Blades become God tier, but the actual user's body isn't like that. It's just a top-tier body without benefits like resisting Devil Fruits.

 

[Impact]

Some supporting evidence for this is that Shisui was extra heavy. That could indicate it was behaving like a powerful soul (souls have weight).

Weight could relate to Haki Mass.

Enma is actively sucking out Zoro's Haki (his soul). That means if true then it is continuously getting heavier.

I don't think we can afford to sleep on this. Zoro has been priming an upgrade for like 200 chapters. It could change soon!

I'm onboard with the idea that the Black Blade is an epic upgrade, but I'm also taking the side of caution with this and proposing those same qualities might not protect the user of the Black Blade. By itself, it's God tier, but with a human wielding it, it has a human limit, and that's probably exactly what Zoro's story is about.

His victory comes from discipline and technique, not because he’s a biological freak of nature like the Emperors are.