We have been baited on the main power ups of the Monster Trio, it has happened to Luffy and Sanji, and I am sure the same is happening to Zoro right now.
Leading to Onigashima, the main power up for Luffy was learning advanced Armament Haki to go through Kaido’s skin, while the power did help, the main power up ended up being weaponizing Conqueror’s haki, and Luffy’s understanding of Advanced Armament might have been essential for unlocking this power up, it was not the final power up.
The main power up Sanji got in early Wano was the raid suit, it gave him a boost to his kicks and speed and pseudo invisibility. But as we know, it also unlocked Sanji’s Germa lineage factor, which changed Sanji’s body powering him up, and all the trauma related to his backstory and the consequences of his actions might have given an emotional boost to develop a way more powerful version of Hell’s memories. Thus, the Raid suit was responsible for Sanji’s power up, but itself isn’t the power up as it was destroyed.
Now, for Zoro, as you guessed it, the main power up has been Enma, and Zoro has been relying a lot on its power (or its ability to channel Zoro’s haki), and it was really effective against Kaido, but, if we go on the inherited will subject, it makes sense Oden’s sword would inherit Oden’s will against Kaido, but now, against King, maybe not so much, but I think Zoro’s time with Enma might have given him whatever he needs to defeat King, but Enma itself will not be the main factor.
Maybe Zoro understanding of Haki by having to “tame” Enma, when applied to both his Sandai Kitetsu and Wado Ichimonji will be what turns them black blades. It makes sense that those weapons are the ones to become black blades, as Wado Ichimonji is the personification of Kuina’s and Zoro’s childhood will to become the greatest swordsman, and Sandai Kitetsu chose Zoro, it belongs to him fully.
There’s also the possibility that he fuses with Enma. Some people theorize that Zoro’s “real” power up was using Conqueror’s Haki, so we have that to consider as well. I think Zoro’s current situation is paralleling Sanji’s myself: his power up is proving to have a drawback of some type that he either has to overcome or bear it’s cost.
What will happen to Zoro? Maybe Enma won’t stop to suck his haki and Zoro will have to put it aside. Or maybe Zoro will still be using it, but Enma in itself will not be the main factor for Zoro’s victory.
Theory by Sotherius (https://www.reddit.com/user/Sotherius)