From BZPB Wiki

The Pandorica was a device created by The Users and housed on the planet Nervissa. If activated, it would cause the destruction of the entire BZPB Multiverse.

Blackout activated it, but, due to everyone forgetting about the "destruction of the entire BZPB multiverse" bit, it just ended up causing the BZPB multiverse to bleed into and become part of the Real World.