HP printers stopped printing black

Both HP printers stopped printing black (6500 All-in-One and 4500 All-in-One). Both connected to Air Port (6500 ethernet, 4500 USB). Four Macs all running latest update version of Mavericks (2009 Mac Pro, 2011 Mac Air 11", 2014 Mac Air 13", and 2014 Mac Mini). All have September 2014 HP drivers downloaded from Apple. Bonjour is set up.

Thought at first 6500 had dried up black cartridge so I switched to 4500. Printed color but no black on both. Swapped ink cartridge in 6500, no luck. Reset printers and reinstalled printers multiple times to no avail. Restarted all Macs, no luck. Restarted Air Port, no luck. Moved printer to USB port on Mini and on 11" Air, no luck. Looked for new driver update, no luck. Printer utilities gave me an error message--what it is I don't know--but test pages wouldn't print black.

Did an auto update occur that whacked out the print drivers? Printers worked fine less than a week ago (2-20-15).