@.access:
I think it's more a matter of how much time you spend feeding the weapon with CoA. Most people will only "harden" their weapons on a few occasions, with the weapons being only part of their arsenal. Only swordsmen rely on their weapon 100% of the time (hence the intimate connection between them and their swords), so we only get to see black blades in their hands.
Vergo's bamboo stick should've been black.
–- Update From New Post Merge ---
CoC being involved in turning a blade black makes no sense. The reason we haven't seen that many black blades is because it is that special. Very few blades and very few characters can actually do it.
It's like "devil fruit" awakenings. Very rare.
It makes sense because, it seem more of a 'put your soul in it', rather than just your strenght.