It gets re-inked in a dark color your printer can still print — every page, every photo, still readable.
HP printers may block printing when any cartridge reads empty. In the HP Smart app, set the black cartridge to “ignore,” or look for ink backup mode.
Press and hold Stop/Cancel while the ink light is on to print in backup mode (varies by model).
Press and hold Stop for 5+ seconds to disable the ink level check.