Blue has an established connotation for marking up text. E.g once upon a time a newspaper editor would scribble notes in original copy before sending to the printers. It stands out against black text but is unobtrusive.
That was a particular hue called “non-photo blue”. It photographed as white when using the common photo-offset printing process. So you could make comments, on the camera-ready copy, that people could read, but that wouldn’t reproduce. I’m old and had proofreading jobs in high school.
It’s camera-ready but might get some comments from a last-chance lookover: “Should this be capitalized?” written in the margin, for example. If the answer is no, it’s OK as is, then the copy can go to the camera without having to be set again. This is more a proofreading stage than an editing stage; that part is long over.
EDIT: Also used for printing instructions, to make sure that things come out in the right order, for instance.
My understanding was in those days copy and paste was a literal thing, so I guess what eventually went to the printer might end up being a frankendoc of sorts …
There is a neat technique called ”stripping in” that lets you cut a word out of the page and replace it with the corrected word from another page. You can use it to correct a misspelling if the length of the corrected word is the same as the wrong one; it avoids having to reset the whole page.
You put the page with the correct word underneath the copy, on a light table (a real one). With an exacto knife, carefully cut out the word, cutting through both sheets. Now the correct word fits exactly in the hole. Hold it together with a piece of white tape (standard supply in all these shops) on the back. The mend is invisible.
It was also the color in which TSR would print maps for old Dungeons and Dragons modules, so that players couldn't photocopy them and pass them around. Functioned as copy-protection.
