Black Eyes of Evil
“...I met this six-year-old child with this blank, pale, emotionless face and... the blackest eyes — the devil’s eyes.”
— Dr. Loomis (about Michael Myers), John Carpenter’s Halloween
Traditionally, a cheap and easy way to show that a character is evil is the use of colored contact lenses in live action, or colored or glowing eyes in animation. Since the 1990s, however, a new trend has emerged — the Black Eyes of Evil.
These days, when someone is possessed by demons, dabbling in the dark arts, psychically dominated by evil aliens, or gone evil in some other way, their eyes will go perfectly black with no whites at all.
Presumably this perfect blackness indicates the purity of the evil within them and their total lack of humanity. Even if a person’s eyes are weirdly colored, there is still a possibility of human connection, but if his/her eyes are gaping wells of nothingness...
One interesting feature of this trope is that these black eyes will return to their normal state if the demon is exorcised/psychic link is broken/etc.