In my case, OfficeJet Pro 6968, it NEVER prints black EXCEPT for the test page that is printed after a firmware downgrade which has 3 sets of parentheses () around each of the 3 colors. To me this means that there is nothing physically preventing (clogging) the black ink from printing -but that the issue is entirely electronic. This started after the latest firmware "upgrade" which I suspect contains a crippling mechanism to prevent older versions of the firmware to be reinstalled correctly. Now I can't print with black ink no matter which cartridge I use. EVen the Copy function and the reports do not print in black. Again, I would suspect a cartridge (although I have tried 3) or printhead issue - except for the fact that it does print those black parentheses cleanly right after the firmware install.