Cook 3/4 cup each blackberries and seedless red grapes, 1/3 cup sugar and 2 teaspoons water in a small saucepan over medium heat until juicy, about 10 minutes. Remove the fruit with a slotted spoon and continue simmering the liquid until syrupy, about 2 more minutes. Let cool slightly. Stir the fruit back into the syrup and spoon over vanilla ice cream.