Iron-gall ink

Iron-gall ink bonds iron salts with tannic acid: it goes down a modest blue-grey and oxidizes toward blue-black as it dries — darkening on the page, permanently. It wrote the Magna Carta, da Vinci’s notebooks, and most of the archive of Western civilization.

Modern formulations (Rohrer & Klingner Salix, Platinum Classic line) are far gentler than the historic stuff, but the etiquette holds: don’t let them dry in a pen, and rinse on schedule. This site’s blue-black field is named for it.