To get this trophy you must press L1 to activate Blacksight and shoot a grenade in the air. You can throw one of your own grenades (doesn't have to be a grenade thrown by enemies). There is a great spot for this in "Chapter 3: Inequalities ; Scene: The Puppet Queen". Load up this scene via chapter select and you should automatically have a grenade in your inventory. Throw it and immediately press L1 to activate Blacksight, then press R2 to shoot it.

It should automatically aim at the grenade, but if it doesn't you can use the right stick to switch between targets. There's also a grenade on a wooden box where this sub-chapter starts. It works with all grenade types in the game (smoke grenades and frag grenades).