Black Blades Need to be Durable for a Strange Reason
Perhaps Black Blades turn black in order to withstand Conqueror's coating at heightened levels. I saw a picture earlier that mentioned Kaido's weapon was particularly durable to withstand the might of his Conqueror's Haki.
Mihawk said something like even the might of an elephant won't break the blade if it's black, but I think all of this talk about swords not breaking is actually inverted. It's not the might of something else that is important to consider, it's the might of the swordsman themselves.
There is a potential for a swordsman to break his own sword by infusing it with powerful Haki. The "swords" that people have used where a particularly powerful person used it have often been explained to be able to allow the user to channel their full strength through it. Such is the case with Whitebeard and Kaido.
I've also seen comments from other users here who suggest that the power-up is simply to create an unbreakable blade. I want to suggest that there is more to it, and that the entire reason the blade needs to be unbreakable is because a mighty swordsman can channel an infinite amount of Haki into it. If a blade can't contain that energy, then the possibility that it will be destroyed as a result of the wielders own actions increases.
