My position here is that Black Blades are exclusive to Armament Haki Specialists. However, with how rare it is for somebody to make one, it can easily imply that at a high level of Armament Mastery a specialist can access Conqueror Coating which is primarily a Conqueror Mastery ability and that through a combination of Armament Haki and Conqueror Haki a Black Blade can be forged.
The evidence may suggest otherwise because it is said a Black Blade is forged over countless battles, and I think Armament Specialists primarily infuse it with Armament Haki throughout that time. They may be able to infuse Conqueror Haki into it as well, at high levels of mastery, but the driving force in a Black Blade is the Armament Haki resonance which leaves parts of its energy in the sword each time it is used. It gradually reaches a point where the resonance is high enough to cause it to become altered as a form of superalloy.
Generally, on this website, people believe that Conqueror Coating is the trigger for a Black Blade to be forged. The reasoning that supports this includes such things as a belief that Conqueror Coating will become permanent in the same way as I described Armament Haki permanently altering the blade.
A counterpoint to this is that Armament Haki characteristics include hardness, and Black Blades are unbreakable. That is not a Conqueror Haki characteristic. In fact, a Conqueror characteristic contradicts this because they can't use low grade weapons. They would break them. A characteristic of Conquerors is that they use so much force that they require a Supreme Grade blade to use the full extent of their strength.
From my perspective, both arguments have merit, but there are some indiscrepancies between them. Conqueror's Haki is mysterious still, so it's not impossible that it evolves the blade, but there are many things pointing to Armament Haki too.
"A counterpoint to this is that Armament Haki characteristics include hardness, and Black Blades are unbreakable. That is not a Conqueror Haki characteristic. In fact, a Conqueror characteristic contradicts this because they can't use low grade weapons. They would break them. "
That is not how this works. The entire point of armament haki is to use your will to shield something. You can use conquerors as armament in order to further reinforce anything. In that case the users Conquerors haki
takes on the characteristic of hardness, because it's being applied as such (Armament)
The literal application of conquerors haki does not break weapons of lower grades. Now that we have more context as to "mass" of haki, what makes much more sense is that a lower graded weapon cannot withstand the "
Full Power" of a character like whitebeard's CoC.
Because the statement says for Whitebeards Naginata "Murakumogiri":
"This naginatas extraordinary durability allowed whitebeard to channel his
full power through it"-
in other words: His full mass
This is then what it also means for Kaidos Kanabo, it can withstand the full might of Kaido's conquerors haki.
Therefore, a conquerors user like WB, can simply choose not to pour his full mass into a lower grade weapon, if he is insistent on using it.
He can still use his conquerors, but at a certain threshold of mass the blade won't be able to handle his haki pouring into it.
So in this example if we use WB, and he was using a lower grade naginata lets say, he could go on to forge that naginata into a black blade,
and then it would be
able to handle the full power of his conquerors, because it would have
unparalleled durability
