User blog comment:Voikus/Black swords/@comment-78.134.87.22-20200610121936

To be honest, since Mihawk mentioned it to Zoro on Kuraigana I automatically assumed that continuous use of Haki on a certain sword will gradually turn the sword/axe/spear/naginata into a Black Sword. Of course, it depends from person to person and probably takes a very long time. Plus, if a sword is used by multiple Haki user I guess that the process is quickened.