Toner fragments have slipped through the cleaning web, which cleans the pressure roller, and re-adhered to paper.
This may occur if:
Printing on both sides of paper
Printing on uncoated (especially rough-textured) paper
Printing a single-dot halftone image
To adjust the following settings, pre-register the type of paper in use as a custom paper. For details about registering custom papers, see "Registering a Custom Paper", Paper Settings.