There is another possibility--
If the printer has "fixed" heads and removable inktanks, (My old Cannon S750
for instance)
The black head may be clogged up with dried ink.
I ended up using Windex, saturated paper towels, and a plastic bag to soak
the head for several days, then rinsed out the ink screen on the head,
sprayed some windex in via the screen, then let the head sit for a day or so
in the plastic bag. Eventually the black nozzles started to unclog. A couple
of additional flushes
with windex and then water, followed by reassembly into the printer, and
execution of the head cleaning sequences restored black printing.