Blindness is a special status effect that prevents a unit from seeing anything. It can be inflicted by several different types of units, including Lookamancers. One way to overcome it is to use a Foolamancer to serve as the unit's eyes. It can be cured by a Healomancer.
Real World References
The incantation for a Lookamancer's blindness spell, "Lemon Jefferson", is a reference to a blind blues singer and guitarist from Texas.