First of all, to understand my point, I think that there are, at least, 2 kind of CoA.
For all we know now the only ability that is more rare than the ability to turn a blade permanently black is future sight. And FS was present to be the acme of a kind of CoO. Therefore it is not far fetched to think that turning a blade permanently black is the acme of a kind of CoA.
This kind of CoA seems to be the one that the swordsmen need to work on to attained the acme of swordsmanship. So it is totally logical that the « Sword God » and the « World’s strongest swordsman » mastered this kind of CoA. It is clearly a proof of strength and of mastery Haki wise.
Why Shanks, Oden, Roger, Whitebeard and others Top tier blade users did not show to have black blade?
Because they are/were not as pro efficient in this kind of CoA than Mihawk and Ryuma. The same way that Katakuri is more proefficient in a kind of CoO than Big Mom, Rayleigh and Kaido for example.
Roger and WB seems to be more proefficient in the barrier kind of CoA.
Now, is having a black blade is proof enough that X is stronger than Y?
No. This is why to prove that Zoro is above Oden, he will re-open Kaido’s wound AND turned Enma black.
He will show that he is stronger or as strong as Oden physically and stronger than him CoA wise.
And the fact that Zoro is weaker than Mihawk means that Prime Oden is weaker than Mihawk.