Blindness

This is a percentile value that determines the actor's disability to see the Player while hiding (sneaking without moving).

0 is the default value, meaning that only the player's skill will affect whether sneaking is successful or not.

100 means that the player can hide right in front of the affected NPC.

This does not make the player completely undetectable as the actor affected by blindness can still hear. But as long as the player is not moving while in sneak, the actor won't know he's there.