Blackrock Pass is the entrance into the Burning Steppes from the Redridge Mountains. It is referred to by some as the "Gates of Hell". By traveling through the pass, you leave behind the tranquil low-level lands of the human starting areas and venture into the scorched and twisted ruin of the Burning Steppes. Low level travelers are advised to avoid Blackrock Pass at all costs, as the enemies beyond are some of the strongest opponents in the game. All enemies are in their 50's with many being elites. (As of patch 2.3, most outdoor mobs in the Burning Steppes are no longer elites, but it is still quite a dangerous place to venture for an unsuspecting Redridge Mountains quester.)
The pass contains a gate that borders Redridge Mountains. The gate is likely a reference to the Black Gate of Mordor in JRR Tolkien's The Lord of the Rings.