In a heatproof measuring cup, combine the cider vinegar and sugar and microwave on high for 30 seconds. Stir until the sugar dissolves. Add the shallot and pepper. Transfer to a shallow bowl and freeze until ice crystals form around the edge, about 15 minutes. Stir the granita with a fork, breaking up any solid pieces. Continue freezing, stirring 1 or 2 more times, until the mixture is icy and fluffy, about 1 hour total.
Arrange the oysters on a large serving plate filled with crushed ice or on a chilled oyster plate. Top the oysters with the granita and serve immediately.