Fun fact: mirrors are actually white. Now let's consider that displays are not mirrors, by their attribute that they emit their own light while mirrors instead reflect it. A 'black' screen is usually understood to be a screen that has had its emission across all wavelengths minimised and so it's most mirror-like property would be the reflection provided by its panel. Displays with reflective glass panels would then behave as a 'black mirror', particularly notable with mobile displays. Now, once you have your black mirror you can take part in the dystopian future depicted in the eponymous television series, or summon demons. Your choice.