Here go the guide on how to discover the dungeon entrance for Blackrock Caverns Entrance.
Blackrock Caverns (Level 80-81)
This instance is entered through the Blackrock Mountain. You can get there from Searing Gorge or from Burning Steppes. Several instances live in this mountain. To get to Blackrock Caverns you need to run around the circle until you find the broken gate. When you run into this hallway you will see some crates scattered around, you go to the right here, and the instance portal is right in front of you.
Check out the two videos below on how to get there: