A water balloon with a rock inside it. Typically used by drunks, frat boys, date rapists and drunken frat boy date rapists. Or the mentally retarded. People who throw black hearts are typically murdered by their targets.
Man: "Those shit-headed frat boys threw a black heart at my windshield and so that is why I cut their testicles off and stabbed them to death."
Officer: "Oh! A black heart? That's understandable then. You're free to go."