The black boxes that can be found in each level unlock new Halo History entries, which goes toward getting the 100% achievement. There are also some unlocked by playing in different Skirmish matches.

Black Box	How to unlock
All Others	Win on each Skirmish map and win Skirmish with each leader
Black Box 01 - Alpha Base	Under the last bridge before entering Alpha Base
Black Box 02 - Relic Approach	Top left of the map, behind a Covenant shield
Black Box 03 - Relic Interior	On the small ramp on the left side right at the start, before going inside the Relic
Black Box 04 - Arcadia City	Just north of the starting point, right next to the skyscraper where you can save Adam
Black Box 05 - Arcadia Outskirts	Go down the first ramp as you are fleeing from the Covenant, it's behind a downed Pelican
Black Box 06 - Dome of Light	Far left of the map, to the left of where you transport the third Rhino tank
Black Box 07 - Scarab	Far right side of the map, in a small alcove with supply crates
Black Box 08 - Ander' Signal	Near the big brute fire line at the start, on a ridge to the right
Black Box 09 - The Flood	Straight out from the base ramp on the other side of the map
Black Box 10 - Shield World	Alongside Bravo platoon, which is the middle platoon you pick up
Black Box 11 - Cleansing	Left rear of the ship, on wings that slant down...you'll need a flying unit
Black Box 12 - Repairs	Left edge of Spirit of Fire, not far from the Power Core building, on the left side
Black Box 13 - Beachhead	On a ledge near the second set of teleporters, near the Covenant base
Black Box 14 - Reactor	Up and left at the top of the first ramp, on the edge of the ramp
Black Box 15 - Escape	Directly opposite the starting point on the north edge of the map, between the Flood and the Covenant base