All of the early TSR Modules (where did the term module come from) used maps on the inside front and back covers. These maps were always printed in a light shade of blue. I have no idea why the decision was made to print them in this colour. The urban legend at the time (the 1980s) was that it made them hard to photocopy. But I would like to think it was an artistic decision.
Blue Maps just look cool. Much better than mundane black and white printed maps. I just threw together a blue map of my own. I find with this type of map a simple program like Paint is best because you want sharp distinction between colour and background white.