Avoid "proper black". Ink isn't "black black" it's just really super dark grey, like a 90% black. The "other" 10% is probably best in the purple range. If you're curious why...just get a cheap felt-tip pen, draw a big splotch on a strip of paper at one end and lay the end into water (keeping the other end out of water). Come back in 2h and you should have some nice colour separation of the tints in the "black" ink.
In older maps the seas often turned into "doodle spaces", notations or other things. I remember seeing a map from Columbus' voyage to the Americas that had drawings of the three ships named in the "spare sea".