Whisk together the sugar and cornstarch in a saucepan. Add 1⁄2 cup of the water, and whisk until combined. Whisk in the rum, orange zest, vanilla bean paste, and salt. Add the thyme sprig. Cook over medium, whisking constantly, until the mixture is thickened, 12 to 14 minutes. Remove from the heat, and fold in the blueberries. Let stand for 30 minutes. Remove and discard the thyme sprig.