There is a Black Box hidden in each mission of Campaign Mode. These locations are listed below. The remaining Black Boxes can be unlocked by beating Skirmish Mode with each of the 6 leaders (Arbiter, Prophet, Cutter, Brute Chieftan, Forge, and Anders) and by winning a Skirmish match on each Skirmish map. Black Boxes unlock text in the Timeline
1) Alpha Base
This one is under the bridge just before the skirmish outside the entrance to Alpha Base. Set your cursor on it and your men will find their way down to collect it via the ramp on the side.
2) Relic Approach
There are multiple power barriers in this level, two with Warthogs behind them, one leading to the end of the level past the enemy base, and one to the northwest, with the Black Box behind it. Beware the snipers in this area. Read more