If this is happening your printer may be dirty.
Check if your printer is able to run a cleaning cycle (print clean page) which can be found by clicking the Windows Start button and choosing Devices and Printers. Right click on the printer in question and choose “Printer Properties” where you may find a tab called Service which will have diagnostic and cleaning utilities.
You may also be able to access diagnostic and cleaning utilities from your Start menu if your printer was installed by using a CD with additional programs and features included.
Note that when more than 1 cleaning cycle might be needed.
If this does not work then try replacing the black ink / toner cartridge.