Bad Vision is a negative status effect in Streets of Rogue. NPCs will be unable to see and will not react to anything that isn't extremely close to them.
It does not completely negate their vision, as you still get spotted approaching them closely and will chase a short distance after you after leaving their sight.
Exact distance it is reduced to is 2 tiles ahead of them.
NPCs will still become hostile if you attack them or cause them to become Dizzy, Slow, or Paralyzed despite not being able to see you.