There is a cargo-cultish love for “old school blue” maps within the OSR. Personally I can’t stand them because my eyes are shitty and high contrast is the best contrast for me (seriously, last time I ran Village of Hommlet* I had to grab Temple of Elemental Evil just to get the black and white map of the village).
(Spellcheck wants me to replace Hommlet with Hotmelt. And you know what? Part of me wants to do that next time I run the adventure.)
And really, the blue maps are low contrast for a reason. They are specifically done in what is known as “Non-Repro Blue” or “Non-Photo Blue” because photocopiers of the 70s and 80s generally couldn’t pick up this shade of blue, acting as an old-school anti-piracy measure. This is why the best graph paper is a pale blue colour as well – so technical drawings on graph paper would photocopy the drawing without the grid.
So, let’s say you have way better eyes than I do and actually want old school blue maps? Take any of my B&W maps and take it into Photoshop (or the Gimp, but I don’t know what the specific shortcuts are there). Hit CTRL-U to bring up the Hue / Saturation adjustment window (Or Image > Adjustments > Hue / Saturation). Select “Colorize” on the lower right, and adjust the sliders… For the images above I used Hue 200, Saturation 55, Lightness 55.
If you want something lower-contrast and almost perfect non-photo blue, then go with Hue 192, Saturation 55, Lightness 72.
And there you have it – how to turn a B&W map into a nightmare that makes my eyes bleed.