It would take a lot of force to cause more than a skin injury from a blunt knife. The skull has been known to deflect bullets when hitting at an angle. It is a tough chunk of bone.

Keep in mind that before you can injure your skull, you need to penetrate the skin. The skin over the skull has a lot of blood flowing through it. Cut it and it bleeds bad. You'd be bleeding awful bad before you ever did any damage to the skull. It would take a strong direct stabbing action to penetrate the skull. The knife blade would likely break first, before any serious penetration beyond just a nick.