Yeah, I worked on the school newspaper with non-repro pencils, now in more modern times they’re all felt-tip pens. Pencils would sometimes write too dark and be reproduced on the ortho films.
But I think the editor’s blue pencil predates this. I’ve seen proofreader’s markups done in blue pencil, it seems to be a tradition amongst editors to mark up the black typewritten copy with blue pencil.