Similar to the Bifrost in Asgard, the Black Bifrost was a means of teleportation used by Malekith the Accursed to travel through the Ten Realms. It was a key element in Malekith's campaign of destruction during the War of the Realms since it allowed him to travel across all the Ten Realms unchecked.
The Black Bifrost was created by the Dark Elves based on the secrets of Bifrost that Loki shared with them. It was powered by magical crystalized mushrooms mined by the Dark Elves in their realm, and it was controlled from Svartalfheim by Bitterhand using the Bitterblade.
In order to stop Malekith's forces from ambushing them again, Freyja put together a team consisting of herself, Punisher; Blade; She-Hulk, and Ghost Rider to destroy it. After fighting their way through an army, the strike team arrived at the Black Bifrost and began assaulting it, but at the same moment, the other Bifrost was destroyed, and with no way to get back to Asgard, they had to use the Black Bifrost instead, so Freyja stole the Bitterblade from Bitterhand and killed him with it. Freyja with her new dark magic powers then sent her teammates to the other realms victimized by the War of the Realms in order to gather allies for a last stand against the Dark Council.
After bringing them all back to Midgard along with Thor; Captain America; Spider-Man; Wolverine; Luke Cage, and Iron Fist, Freyja attempted to destroy the Black Bifrost, but was wounded by Malekith. Odin arrived and together they fought off the army before sacrificing themselves to destroy the Black Bifrost.