In a medium saucepan over medium heat, combine 3 cups blackberries, ¼ cup sugar, ¼ cup water and 3 tablespoons freshly squeezed lemon juice and bring to a simmer. Continue simmering for 1 minute. In a small bowl, whisk 2 teaspoons cornstarch into 2 tablespoons water. Add to the blackberry mixture and continue simmering, stirring frequently, until thickened, 2-3 minutes. Season to taste with salt and cinnamon. Let cool, then serve over ice cream or yogurt.