Black Operations is the Black Market Contraband stream that is unlocked after the completion of an Operation. Black Operations is entirely comprised of Reserves, which contain random cosmetic items, similar to Supply Drops. During Operation: First Strike, it was unlocked at Tier 201, but has since changed to Tier 101 since the release of Operation Absolute Zero. With Operation Spectre Rising, the unlock tier has been lowered to Tier 51.
Operation Grand Heist, at launch, had essentially removed Black Operations from the game, as it was impossible to earn Reserves, or any item when Black Operations was attained. This has been since changed, as when reaching Tier 101, the player will start earning Reserve Cases.