Graphic arts film wasn't at all fussy about the shade of blue (as you note) and so while there were expensive non-repro blue markers and pencils, everyone I knew (at the very end of the era of graphic arts cameras) used blue highlighters, so design studios were full of them.

I've stuck with blue as the only highlighter colour I'll ever use, more than 20 years since the original rationale.

Also, used to freak people out scribbling (non-repro) obscenities on a flat that was going to be sent to photo and turned into a newspaper the next day.