That's annoying!! All the expectation and build up and then you get lighter...
Well, I agree with Walker it does sound like the PiezoFlush is still mixed in with the ink either in the ink lines or the damper.
1) Printing photo images does not move that much black ink because our curves do not print that much black ink.
2) Your original intention in flushing it out by printing it with the special flush target in Calibration mode will produce unavoidable intermixing in the damper. The PiezoFlush is better at displacing ink in the damper than ink is at displacing PiezoFlush once it is in the damper. They sort of swirl around each other for a bit. So you may just not have printed with the undilluted ink yet.
3) I read that you said the nozzle check looks normal, but what you are describing could also be caused by no black ink printing and what you are seeing is not diluted black ink but rather the underlying shades 2, 3, 4 which under-print black ink with our curves.
4) If you print the normal calibration mode target in calibration mode you can verify if the black ink prints normally. You will see if it prints the 21 steps at the same time the other shades print 21 steps. If all or part of it is missing- then that's going to take some trouble shooting, but it also might indicate the cart is not primed properly, air is coming though the print head, and repriming is an easy fix.
5) I know you must hate wasting the other ink shades by Power Cleaning - but what we would do at Cone Editions Press is just get it over with and run 3-4 Power Cleans to suck out the ink from the ink lines and damper and have that replaced by the new ink. It's sure bet way to do ink changes especially when time is incredibly valuable or we need to get a print job out that day. There's no messing around with doing it by the suction created by the Power Clean. It takes a minimum of 3 for what ever is in the cart to reach the print head. We do the 4th for good measure. These are equivalent to the INITIAL INK CHARGE. Also toping off and shaking all the carts prior to running the Power Cleans insure freshly shaken ink is in all the ink lines and dampers after you are finished purging the printer.
Please let us know what you find out. It is killing me to read this. I still jaw drop looking at matte prints with the Ultra HD. I hope to read you have as good a reaction as I still do.