Maybe you need an extremely strong attachment to the sword or either way peak sword skills. That will be somewhat of an excuse for Zoro and Wado and the only other 2 people we know to have/ have made black blades: Mihawk and Ryuma, WSS if their time and probably best in history exactly because they have made their black blades.
The problem now for me is that what have black blades that makes them special? Only the feat if making one seems something really exceptional but actually, about the sword, Zoro had one, Shusui, for more than 2 years. That sword should be something unique aling with Yoru which is Mihawk blade but in truth Zoro just returned it to Ryuma without the sword never showing anything special (it is only extremely durable if compared to ither swords, but that was an info we got pre haki, was it Thriller bark arc?) and on the contrary Enma, the sword he get in place of Shusui, seemed a monster sword in a way (for sure looked way better than Shusui, the Wado or the Nidai) despite not being black.