If this is happening your printer may be dirty.
Check if your printer is able to run a print clean page which can be found by going into printers and faxes.
Right click on the printer in question and choosing printing preferences where you should find a tab called Service.
If it is a HP laser jet then you will either have the HP toolbox software installed which you can access a print cleaning page from or from the management console on the printer itself where a print cleaning page can be accessed.
More than 1 print clean might be needed.
If this does not work then try replacing the black ink cartridge.