Black Mirror is a british sci-fi TV series in the form of an anthology of stories, much like in the same style as Twillight Zone and The Scary Door, which I bet is a real show, each episode commenting and satirizing our dependency on technology, entertainment and virtual interaction, concluding on a depressing note, generally. Bare in mind that even when it ends with a closure, it's bound to be questionable or bleak, but that's exactly what makes this show great.
If you're asking yourself, yes, there is an adventure video game called Black Mirror, but it's not related. There's also a 1981 movie with the same name, again not related. The title "Black Mirror" is a metaphor that's supposed to reflect (pun intended) the screen of your TV.