Why does some black test come out blurry when the rest is good?

Chances are there's a build up of ink at the ink cartridge. It’s also possible that the ink cartridge is defective. Remove the black cartridge, grab a few cotton swabs. Drip a few drops of water onto the cotton swab and wipe the copper cartridge contacts.
Dampen a lint free cloth and wipe the bottom of the ink cartridge. (This will free up and remove any ink build-up). Re-assemble and print a test page. If this does not help use the printer’s software and run, "Clean the print heads". If using Win XP left click once on the Start button. Slide up to and left click on Printers and Faxes. Within the Printers and Faxes window right click once on the printer with a little black check mark next to it. From the pop-up list, left click once on "Properties".
Than left click once on Printing Preferences. Than, Left click once on Services. Clean the print heads here.

Hope you find this helpful!
Mike