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The black ink is, no doubt, not printing. (Sometimes this could be owing to a quirk, or a corrupted line of code, in the printer's driver software though.)
Try printing from a TextEdit or a Notepad document, a page or so. If it comes out blank, the black ink isn't printing for sure.
How new is the printer?
Thanks for the information.
What happens when you send the printer a black-text-only print from either TextEdit, or Notepad ?