He needs Awakening to use Haki on the scrap, hence why he has not shown it.
Why does he need awakening to do this?
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Anyways, not all CoA users have good enough ryuo to use black hardening. Zoro is the only supernova, to my knowledge, who has actually used black hardening on his weapons. Does this mean that other supernova don't have haki? Obviously not.
Drake is a confirmed haki user. He bit caribou in the cover story. Yet he has never blackened his weapons.
In order to take out Killer, Zoro needed to use black hardening on his blades. This indicates that Killer must have some degree of CoA even if his blade wasn't hardened black. Otherwise, why would Zoro need to use haki to defeat him?
Law has shown black CoA but only on his arms, he has never hardened his sword. Does that mean his sword isn't imbued with haki?
You can even look at Tashigi. She is a confirmed haki user, but never hardened her sword black like zoro did.
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At worst, Kid just has bad ryuo. At best, Kid actually can use black hardening on his metal, as he was shown doing so in stampede(a movie where Oda was a creative supervisor). Maybe he hasn't done it yet because Oda is saving it for later, idk.