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The black ink may have run dry, or it may have become clogged, too. Has there been very little use of the blank ink color lately? or has the printer been altogether disused for some time lately?
How old is the printer?
Have you checked the ink levels?
When an inkjet printer has this sort of problem, doing a purge (or three) of the black ink system, or the whole ink system, is a usual thing to try. Otherwise known as a printhead cleaning.