Z3100 #1, where solid black was printing as midnight blue.
Solution: Pulled MK/R printhead. Totally mucked up with dried and gooey chunks of black. Red (which is not really red, but more of a salmon color) was fine. Thoroughly cleaned the head and re-installed. Voila, problem gone; 0,0,0 now prints on canvas as solid black. Don't know about the dark blue, but at this point I don't care.
Z3100 #2, where reds were printing as rust or dark brown.
Solution: Pulled MK/R printhead. Same gooey and dried black ink problem. Seems MK ink was getting leaking into R side, causing reds to print dark. Following a thorough cleaning, problem is now gone.
What I find so odd is that during the 4+ years I've owned these printers, the only color problems I have had involve the MK/R channels. All other printheads in both printers are still the originals that shipped with the printer. But, in both printers, I've replaced the MK/R printhead twice.