@Ailantd@Curator Easier to remove if not black or dark.
Some shades or "non-photo" blue worked well with special reproduction cameras. Magic not involved. With color scanners it would be a defect if it skipped some blues.
Nowadays it makes sense to draw lines with your colored pens along with black and then scan it with your own scanner and then see which shade is easiest to remove. Could be light blue, could be something else.