If you're talking about the Black Mirror game from 2003, be aware that the North American boxed version was infected with StarForce DRM and will not work on anything later than XP unless you can find a working crack.
There was a reissue version that was bundled in a set of games that used LaserLock instead of StarForce. The LaserLock version might or might not work with current versions of Windows and I believe it was only sold in the UK.
If you're talking about the 2017 Black Mirror, which is a completely different game from the 2003 Black Mirror, there was never a boxed version for PC. There was a PS4 version that came in a DVD case.
GOG sells both the 2003 and the 2017 Black Mirror games, and will probably have a good sale around Christmas. I've seen the 2003 version on GOG for as little as $1.99 during an 80% off sale.