The last image, with the black background, was helpful to students who did not yet know how much information exists within shadows. I prefer the chromatic black effect of the second to last image (and that is the state which saw a full edition), but full black layer in the last image was a helpful aid in discussing how tone shifts as each new color/tone is placed up on the image. It does reveal that the other darker layers in the earlier state were made with transparent green.
(update: I made a set of instruction videos for my students. Here is a link to the post http://artandwater.blogspot.com/2017/03/multi-color-multi-block-print-you-can.html)