[Inziladun]

I'd like to add that perhaps any blade used by the Nazgûl to actually physically wound someone in the "light" world might be expected to vanish.
Since we see that a blade piercing a Ringwraith breaks and is destroyed, maybe contact with the truly living has a similar effect on "dark" weapons. Why this might be is uncertain, but maybe it's related to the nature of the Nazgûl, described as they are as hating light and living things, yet being attracted to them. The Nazgûl existed as extensions of Sauron's fea, and he, being a negative spirit of the dark, might impart some sort of anti-life/anti light into those who share his will and soul, which might filter down into specially made weapons intended for their use.