Non-Photo Blue pencils are a particular shade of blue that could not be detected by a graphic arts camera like a photostat camera. I ran stat cameras in the era when graphic design was an analog pursuit involving rubber cement, illustration board, razor blades, rubylith, waxers, zip-a-tone and a lot of manual labor. It also involved Non-Photo Blue Pencils which I have a bunch all about my house. I do a lot of my initial sketches with these pencils and work over top of them with graphite pencils or ink.
I was afraid that these pencils were going to disappear. A few years ago I asked a clerk in an art supply store for non-repro blue 2mm leads for a clutch pencil. Her response was, “What do you mean by non-repro blue?” I thought my days of enjoying these pencils were numbered. Fortunately, that’s not the case. They are not necessarily easy to find, but non-repro pencils and leads are still available. The Caran D’Ache at the bottom of the photo is a recent acquisition and the Swiss make a very nice pencil!