How to complete Dark Ops Challenge 4 in Blackout (Battle Royal) in COD Black Ops 4. This Challenge is called Open the Blast Doors. To complete it, go to the Fracking Tower and find the blast door located on the floor in the center. A button that opens the blast doors can be found just north of it.
The Blackout Mode in Call of Duty Black Ops 4 has 9 Dark Ops Challenges. None of them have a description or location attached until you find the actual challenge and complete it. Once you complete a dark ops challenge you unlock a new calling card.