I've just found another tutorial for making a black wash, in this case primarily for terrain rather than figures:
There's a lengthy preamble relating to an earlier video, but the instructional part begins at the three minute mark. For a general summary, the accompanying text reads:
Mix flow aid into water at approximately 10 (water) to 1 (flow aid). Then mix water and matte medium at approx. 60/40 water to medium. Add ink until you achieve the desired results.