Using Linux 12.3 and printer HP deskjet 920c.
Used to work satisfactorily.
But all of a sudden, in the middle of a printout, the black colour faded and died out. HP-toolbox said level of ink low (but not empty).
Spent part of my fortune and bought a new black cartridge for the printer.
Result: Colour printing OK - with the exception that black colour is still missing.
Used bad language for a while and then I changed OS to Windows XP. Nice printing, all colours, including black.
Changed to the old black cartridge. Nice printing, all colours, including black.
Returned to SUSE Linux. Neither black cartridge gives any black printing.
Searched my attic and found an old Epson Stylus Color 440 (not used for 5+ years). After cleaning, and with the printers old cartridges (!) it printed colours - but not black.
Reverted to Windows XP. Printed colours, including black.
Borrowed an Epson Stylus SX440W: same result, no black printing from Linux, including black from Windows XP.
Uninstalled printers and reinstalled. Same result.
Removed CUPS and installed CUPS again. Same result, colours minus black.
How do I proceed???

Is anything interesting shown by "rpm -Va" ?

What are you doing to print? "lp filename.txt" , a print option from a word processor, etc ?

Have you looked at logs such as /var/log/messages ?

"rpm -Va" gives an extensiv list telling that I'm missing help and man files in languages I don't need for mainly programs Tomboy, wget and vlc. What should I actually look for?

I'm using print options from different programs like LibreOffice or different picture managers. I tried the "lp filename.txt" and the result is the same; black text is just a shadow of gray, partly impossible to interpret if you don't know on beforehand what is written.

var/log/messages just gives the timing and "Started CUPS Printing Service", no remarks about problems or failures.

Does this give a clue???

Not much clue there but at least you've proved it's not all caused by a setting in your word processor. I wondered if a config file related to printing had been altered.

Does cups have the correct type of printer in its records?

Tnx linosaurusroot for trying!

Yes, CUPS has the correct type of printer in its records.

I checked "GNU/Linux Basic" e-book by Joaquín Lopez Sanchez-Montañés et al in the Free Technology Academy Series. In Chapter 11.9 Configuring Printers he writes:

I tried this in vain - until I realised that there is a printing error. It should read http://localhost:631 and then clicking the Printers tab. And Voilà! it says that I have Epson-Stylus-SX420 and hpdeskjet920c with status Idle - "ready to print". When opening this printer I have some options; Maintenance being one of them; so I chose "Clean Print Heads" - which produced an error message: there is no command Clean. As I previously have installed hp-toolbox I checked hp-toolbox. No problems with cleaning cartridges. Prints out 24 squares in different colours - but black is totally absent.
Back to CUPS website; another option is "Print Test page" - so OK here we go. Nice printing; colours including dark grey/black - on an area of approx 2 x 6 cm at the bottom part of the paper but the remaining part is white - well, there are hints of a headline "Printer Test Page" and probably the intention was to print a frame around the page as well.
Another option is Print Self Test Page. This option jams the printer and produces a white paper...

I am considering reverting to my childhood's fountain pen (with manual filling)... or will OpenSuse 13.1 solve the problem?

Installed SUSE Linux 13.1 (on the machine with the printing problem, not the one I am using right now where I because of laziness sticks to 12.3). And the problem disappeared. Black is back in business...