Darkness

Demons and other monsters capable of creating magical darkness can see through their own darkness and the darkness produced by allied monsters.1) This is not explicitly stated anywhere to my knowledge, however it only makes sense. As a strong example, note that the Type VI demon (Balor) “sheds” darkness in a 10' radius, implying that the ability is meant to be used in an area around the monster. Even with 75% magic resistance, one cannot imagine that the intention was for the Type IV to blind itself 25% of the time.