"...I met this six-year-old child with this blank, pale, emotionless face and... the blackest eyes — the devil's eyes." — Dr. Loomis (speaking about lil’ Mikey Myers), Halloween (1978)
Ah, Dark Willow. A cheap and easy way to show that a character is PURE EVIL is the use of colored contact lenses. This trend became popular with shows like Supernatural (2005) and Buffy the Vampire Slayer. Also, pulsing black veins to signify rot and decay. Of course, if the demon is exorcised then all the actor or actress need to do is remove the contacts, wipe off the make-up and presto, they’re back to normal. Evil is, once again, thwarted.
Of course, this trend only dates back to the 1980s (I think the 1985 campy Italian schlock-fest Demons was the first to use it) because in the 1970s, to signify, “that kid just ain’t right,” actors would use yellow contact lenses, bonus points if the Special Effects Department could get them to glow:
Also, open sores and British teeth. Or, in Robert De Niro’s case, comically long fingernails:
So, in short, the whole idea of demons with black eyes comes from cheap Pop Culture, which seems to be where most people get their ideas of theology, esotericism and the divine.