Blue-black ink darkens by the same reaction that once blackened teeth in Japan
Words written in blue-black fountain pen ink sink towards black after a while. Nothing is fading; a chemical reaction is running on the paper. And that same reaction was once used in Japan to blacken teeth.
An ink made of galls and iron
Iron gall ink is built from tannin and iron. The tannin comes from galls, the growths that insects make trees produce. Mixed with iron sulphate it forms a pale, water-soluble complex; exposed to air it oxidises into a dark compound that no longer dissolves. In Europe this was the standard ink for some 1,400 years, from the 5th century to the 19th. The 4th-century Codex Sinaiticus and Leonardo da Vinci's drawings belong to the same family.
The reaction is slow, though, so on its own it would leave freshly written strokes unreadable. Makers added a blue dye that shows immediately. Blue from the dye first, black from the oxidising iron later: those two stages became the name.
Ohaguro ran on the same chemistry
Ohaguro, the Japanese blackening of teeth, meant applying two things to the teeth in turn: powdered fushi galls and kanemizu. The galls grow on the sumac tree and are about 60 per cent tannin; kanemizu is a solution of iron(II) acetate. The iron(II) is oxidised to iron(III), binds to the tannin and becomes ferric tannate, black and insoluble. That is how the Japan Dental Association explains it. Paper or tooth enamel, only the surface was different.
The ink that eats paper
Acidity has its price. German has a word for the decay, ink eating, and it runs from hairline cracks to open holes, worst where the ink is laid on thickly, as in musical notation. To protect Bach's autograph of the Well-Tempered Clavier, Book II, written between 1739 and 1742, the British Library washed the leaves, locked up the iron with calcium phytate and stabilised them with calcium bicarbonate. The method came into use in the mid-1990s.
Still on the shelf
Iron gall inks never died out. On the official spec sheet for Platinum's INK-1200 bottles, #1 Black and #2 Red are listed as alkaline and only #3 Blue-Black as acidic. The six Classic Inks of 2017 carry the darkening into colours other than blue: cassis, forest, citrus, khaki, sepia and lavender, each of them ending in black. In Britain, iron gall ink is still used in the registers of births, marriages and deaths. It does corrode metal, so do not leave it sitting in a pen.
Sources
Preserving Bach's manuscripts (British Library)
Bottled fountain pen ink INK-1200 (Platinum)
On ohaguro (Japan Dental Association)