Hi macclad, I hope you were able to find the information you were looking for in that alt-photo discussion; it went off in several different directions, as I recall, but just in case you didn't, the quick answer to your question is that Steichen most probably used a positive rather than a negative to print the cyanotype layer, in order to get that deep cyan between the trees, and that he most probably colored in the moon by hand (note there's no corresponding moon reflected in the water).
Happy experimenting,
Katharine