Roger and shanks would have it if it was coc.
It's something to do with loving your sword very much.
Idc about excuses for failures who couldnt make a black blade.
Zoro had Wado for 20 years, Sandai for 2 years & Enma for 2 months.
He's been putting his Haki on his swords for 2 years now, except Enma which is still at 2 months of usage.
If putting Haki on blades for a duration of time is the criteria... its stupid to believe that Zoro will turn 3 blades black in multiple different fights, cuz Oda is not writing Zoro getting the same feat in 3 different fights,
The only common thread... is CoC. Which Zoro only started using on all 3 since the King fight. Same Haki, and same duration. Put 2 + 2 together.