I agree with Graham but you state "When printing a Word or Excel document using black only".

Why have you only mentioned "Word or Excel"? Does it happen if you print a pdf document or something from the web "using black only"?

Printer model and how old is it? If it has been used a lot it could be at the end of its service life and need a new maintenance box installing but in this case I would have expected you to have seen warning messages pop up.