They simply do not use their sword so often. To have a black blade, it is necessary to have long and hard battles blackening and using the sword efficiently.
WB didn't even leave his black blade to collide with Oden, he just doesn't use his blade much, much less blacken it.
Same thing is Shanks and Big Mom, they don't use their swords much, they don't blacken their haki sword very often just like Mihawk, Ryuma and Zoro.