I wonder if you may be dealing with some air in the ink line or damper, or it may coincidentally be that your black damper has too much particle build up which is restricting ink flow (the two blacks use the same damper and print head channel, but have different ink lines). Have you tried manually cleaning the printer's capping station, wiper blade and bottom of the print head?
Please remove the MK refill cart and check it to make sure it's not leaking from anywhere, the air inlet point is open, cartridge body contains enough ink, and exit chambers are at lest 1/2 filled with ink for proper flow. If all looks good, reinsert it snugly into your printer. After the printer pressurizes the carts and says "READY" on the LCD panel, do 1-3 regular cleaning cycles with the printer in MK mode, then print a nozzle check to see how things look. If the black still isn't printing, and you haven't already cleaned your printer, I recommend you clean the capping station, wiper blade and bottom of the print head following our instructions, here: http://www.inkjetmall.com/tech/content.php?133-Printer-Cleaning-and-Preventative-Maintenance
Please keep me posted, and let me know your results, if you have questions, or there's anything else I can help you with.
Best regards~ Dana