The act of when you are tramatized by a huge black dick and can't function in society. That is forever what you are fixated on and can't stop thinking about it.
In fandom, a Little Black Dress is a character who someone feels can be "shipped" (paired up in a relationship) with virtually any other character. A Harry Potter fan whose Little Black Dress is Hermione would not only read Ron/Hermione fanfiction, but also Harry/Hermione, Snape/Hermione, Ginny/Hermione, and so forth.