This weapon was introduced with the Mann-Conomy Update. It replaces the normal Rocket Launcher in the primary item slot.
When the opponent is hit by a rocket from the Black Box (either a direct hit or splash damage), you will gain 15 health per rocket. However, the Black Box can only hold three rockets per clip.
The Black Box is a part of the Tank Buster set. If it's equipped with the Grenadier's Softcap and the Battalion's Backup, you will gain 20% resistance from damage from sentries.