I could see one of the prerequisites for forging a black blade being conqueror's haki. That would explain why it's so rare. But I doubt it ends there, because the issue is this: Roger was most likely the strongest user of conqueror's of all time, that's literally what his character stands for, to conquer the seas, the "king" of the pirates. Yet he didn't have a black blade. So that likely can't be the sole or even biggest factor in deciding it.
The reason I can see CoC being one of the prerequisites is because there is a common theme here with how Ryuma was stated to forge his black blade, and how CoC works.
Notice how CoC can't be trained but gets stronger through natural growth, similar to Ryuma who is said to have forged the black blade through his "history" of battle, so it's not something you can just figure out one day, train, and be able to do. It's over time with which it occurs, as you likely grow with your blade.
That's not to say the highest level of CoC will guarantee your blade is black, but if you don't have CoC, you by default will not be able to turn your blade black. Perma hardening your blade even past your death strikes me as the qualities of a conqueror, because if you think about it the hardened sword embodies the users spirit (which is what CoC is at its core).
Ofcourse haki blooms while in extreme conditions/ battles as well, that goes for any type of haki. So CoC doesn't have to be involved. I still think armament as far as haki goes is probably the key here to turning your blade black, I also believe overall familiarity with the blade, so love for the blade or time spent with the blade in battle, is also a factor (very possible Wado Ichimonji will be the first blade Zoro forges black).
At the end of the day I think mentality is what determines this, because at a certain point we know haki is maxed out so why didn't Roger or WB turn their blades black? Maybe because they weren't as obsessed or as dedicated to their blades like Mihawk, Ryuma or Zoro. That's what haki is to begin with, it starts with a will to do something or belief in something. "I believe my hands are as hard as steel, therefore they are" kinda deal. (Big Mom who is usually the most durable character, when distraught she was bleeding from her knee simply touching the ground). Someone like Mihawk is probably much more focused on their sword than Roger, like a nick on his sword would be a disgrace to him, his energy and focus probably goes in preserving his blade, this will to enforce/protect his blade could have manifested into a perma black blade through time.
I don't think it's a good argument to say that Roger doesn't past the quality test for haki, if anything it's Roger who does, more so than anyone. This is why I think it's a combination of maxed out haki + mentality. Roger might just lack the mentality.
Tldr; I can see CoC as a prereq for forging a black blade thanks to it being the most linked to natural progression vs instant trigger blooming. (As you grow stronger through battles, your blade grows stronger, your haki grows stronger etc.) but that can't be the only factor here.
The reality is no one knows, because Oda himself is just figuring these things out as he goes.