What's the point of the black blade when Zoro can already cut anything & one shot anyone in the verse? I wonder if it has more properties we don't know about, other than being even less likely to break (which again don't really seem necessary, unless it's needed for some reason against Mihawk).
Shusui was a black blade, but Zoro never acknowledged it the way he did with Enma, and of course he wasn't the one to actually forge it. This all makes the grim reaper stuff (seemingly connected to the swords like
@yorosenpai pointed out) even more intriguing. Is Oda going the bankai route and we might get some weird (and pointless) abilities? What could Zoro possibly gain from all of this?
It could always simply represent the ultimate mastery of swordsmanship, especially if the will fulfilling theory ends up being right, because he'd already have to accomplish something exceptional in the first place to obtain it... but I don't think so.
Imagine if a swordsman who doesn't have a black blade needs one to stand a chance against one who does. I know it sounds a bit ridiculous, but at the same time we only know of two black blade users, one happens to be the current King of Swordsmen and the other one was known as the God of the Blade. Think of the shitstorm that would follow.
Zoro is honestly way too strong right now. No one can compete except maybe Mihawk. He broke the story.
@nik87