Pour blueberries into a medium saucepan with sugar, cinnamon, and 1/4 cup of water, set over medium heat, and bring mixture to a simmer. In a small bowl, whisk together the cornstarch with 1 teaspoon of water. Add this slurry to the simmering blueberries. Keep stirring until the blueberry sauce thickens, 1–2 minutes. Allow the mixture to cool slightly.