I’ve been dipping into a book by Franz Reitinger called Die blaue Epoche: reduktive Farbigkeit im Rokoko (2016), which explores the fashion, between about 1730 and 1765, for printing mezzotints using blue ink. It wasn’t something I’d really been aware of before, but I have always found the colour, Prussian blue, quite captivating. I only need to think of the paste paper on a copy of Adam Smith’s Wealth of Nations which I encountered back in 1998 (a paper which started my interest in decorated paper, and which I wrote about here); there is just something about that wonderful deep blue. When you see it used on mezzotints, the images almost seem to lift off the paper on which they are printed.
I’m currently cataloguing a little collection of such blue mezzotints. If may be interested, then by all means do get in touch.