HP OfficeJet, Deskjet won't print black

I have a HP 4215 OfficeJet that suddenly won't print text in black. I changed cartridges, ran HP diagnostics and cleaning utilities, installed updated drivers, all without success. It will print test pages fine but nothing from Word, Notepad, WordPad, or OpenOffice will it print in black. I can change the font color slightly from black toward gray and it will print and it will print all colors fine.

Thinking the printer had died, I dug out a HP 1320 DeskJet that I bought on closeout and never used, put new cartridges in it and plugged it up. Same EXACT thing! I have tried every setting, diagnostic utility that HP has, re-installed drivers, checked for updates, nothing has helped. To restate, I can change the font color very, very slightly from black toward gray and it will print. It just won't print in pure black.

I see a number of similar posts here on tom's and across the 'net but no real fix. Surely someone out there has read or heard of a solution. I am sure it is a simple one that I am overlooking.

I am totally baffled. Help!