OK, curiosity wins... I admit I have no idea what "dark reaction" is supposed to be... Tho I'll add that I have read more "gum" literature than is probably good for me, and never came across (or anyway noticed) the term until lately on this list. I am reminded however, that when I first started gum printing, I did, or tried to do, what "everyone" seemed to do, which was mix up a batch of each color in baby-food jars, and use for printing until it ran out.
I don't in fact know HOW they did that, because in my jars it dried up almost immediately and/or the effort of stirring it into usability was far greater than the effort of mixing again.... True, I don't usually insist on repeating a mix EXACTLY, as gum can be fine-tuned in development, or wiped out, or other... (but then I never confused gum with the esthetic of platinum as seems, alas and alack, to be a pitfall, seduction, tendency, temptation [whatever noun you prefer] around here of late). In any event, generally speaking, my notes on a mix let me match it, at least well enough...
I realize "dark reaction" could be something I call something else... But if someone will define it, my thanks in advance.