It's a real thing: in ancient times when I worked on a yearbook staff, we used non-photo blue markers to mark up the physical pages we sent to the publisher. I

It's a real thing: in ancient times when I worked on a yearbook staff, we used non-photo blue markers to mark up the physical pages we sent to the publisher. I don't know how these pre-digital reproduction systems excluded the blue, nor to I know if this system is still in use in the digital era.