Any teenage girl who dresses in dark clothing automatically knows the Dark Arts by proxy.
A lot of comedies/dramas/urban fantasy movies need some way for magic to leak into an otherwise mundane world. Some times it's through a Gypsy Curse, or a magical wishing trinket, or the brain fart of a passing genie. But if you need a reliable human factor for this magical working, you can't go wrong with a Goth girl. After all, they wear black and occasionally have pentacles! That's what magic is all about, right? This trope seems to be especially popular in films and other fiction since the release of The Craft, but its routes lie in real life teenage rebellion. If a teen girl wants to rebel against the status quo, it's likely they might a) go Goth, b) take up some form of paganism, or c) do both at the same time. Fiction tends to take this farther, with Goth girls automatically casting hexes and flushing the Rule of Three down the toilet.
Examples of this trope include: