a black blade shouldnt be used to show why one is stronger than another person but it should be a net positive not negative.
black blades regardless of how they are made increase the rank/level of a blade. It has more to do with strengthening the blade itself than the user. It can be passed on to another user and that same person would experience the same result. Just because one possesses a black blade does not mean x person is stronger than y without one. However it also means if y person should make a black blade of their own, then they are stronger for it because now their blades are even stronger/better than what they had before . stronger/better blades = more destructive attacks.
exhibit A : zoro had a black blade all throughout pretimeskip and was a lot weaker than other swordsman
however zoro with a black blade prets is significantly stronger than zoro without one.
as to why oda wants zoro , ryuuma , mihawk to have one and others dont looking moreso like its something he likes to be unique.
mihawk isnt considered stronger than other swordsmen because he has a black blade but due to the title the author has decided to give him.