Black Ball Rain is a variant of Continuous Black Power Ball in the anime and the energy barrage variant of the Combined Black Power Ball in the manga used by Goku Black in the anime and Fused Zamasu in the manga.
The user charges two Black Power Balls in each hand and releases them in the form of multiple blasts in the sky. The energy blasts come to the ground, creating huge explosions. Anyone caught in the explosions will receive massive damage. It also possesses a stronger variant when the Combined Black Power Ball is used as an energy barrage.
In the anime, Goku Black used this attack to try to get Future Trunks in his line of sight. However, Future Trunks luckily took cover as this attack ravaged parts of West City. In the manga, Fused Zamasu is the one to use this attack, albeit a stronger variant, after blasting Goku and Vegeta away, he announces himself as the Supreme God as he unleashes this blasts to show off his power. Goku and Vegeta managed to evade most of them and Goku blocked all energy blasts heading towards Shin, Gowasu, Future Trunks, and Future Mai.