In a cup or small bowl, mix together the arrowroot or cornstarch and 1/4 cup cold water. Stir the mixture into the blueberries. Simmer, stirring often, until the berries have broken down a bit and the mixture has thickened, about 10 minutes. Remove from the heat. Cool, transfer to a jar or other airtight container, and refrigerate until ready to use. Reheat as desired for use on pancakes, oatmeal, cheesecake, yogurt, etc.