Sometimes said to be the ink of the Old Masters, however there seems to be a difference between walnut ink and "iron gall ink" made from oaks instead of walnut. This is not our specialty so we'd refer you to art history specialists here. Walnut Ink is usually made from the Juglans nigra tree and used as an ink.
When discussing dyes, there are different standards for lightfastness than there are in terms of archival art supplies. In her book Natural Palettes, Sasha Duerr mention that Black Walnut Husks "create stable and lightfast and washfast dyes, from deep dark browns when used on their own to solid black when iron is added." As a dye is an interesting option, though she cautions about walnut allergies.
In terms of archival standards for artwork it's considered to have poor lightfastness.
