What eventually worked for me was performing a HARD RESET as explainted in step 5 below after completing the other 4 steps first.
I experienced the same problem with my HP 6500A Plus all-in-one printer and here is what eventually worked. If you have
1) performed a soft reset by pulling out the power cord from the back of the printer while it is still on then reconnecting after 15 seconds.
2) Cleaned the printed head (if it is removable, lift the lever and run the printer heads under warm running water then thoroughly dry.
3) You may need to take a tissue of lint-free paper towel, dab some warm water on the end, and hold it over the ink release portion of the cartridge before reinserting into the printhead.
4) Replace any empty cartridges. HP Printers are notorious for not allowing a print job go through if an empty ink cartridge needs to be replaced.
5) What ultimately worked for me after completing the above steps was performing a HARD RESET. While the machine was still on, I pulled out the printhead and ink cartridges (also cleaned them) then pulled out the power cord from the back of the printer while it was still on. After waiting 15 seconds, I replugged the power cord into the back and waited for the error message: "printer error with printhead/catridges". Next I put the printhead back, then the ink catridges, which forced the machine to go through an intialization and alignment process. It took at least 10 minutes for it to cycle through. I printed a test page and saw it finally printed black (I already present it to print black ink only).