Dell Photo 926 won't print black

I have replaced both black and color cartridges and it still won't print black.  It is so difficult to determine if it is the black cartridge with the probleme or the (I beleive) it  is the black ink in the color cartridge.

PLEASe don't suggest for me to clen the cartridge heads.  I really don't think that is the problem.  It was doing this before I replaced both cartridges!  I have already reinstalled the software.  The biggest clue/problem is that on the test page and the clean cartridges test page, the black bar does not print at all.

thanks