1 pint blueberries, picked over and washed
2 cups water
1/2 cup sugar, plus more if needed
1 teaspoon ground cinnamon, plus more if needed
1 cup plain yogurt or sour cream, plus more for garnish
Combine the blueberries, water, sugar, and cinnamon in a medium saucepan and turn the heat to medium. Cook, stirring occasionally, until the blueberries fall apart, 10 to 15 minutes.
Cool the mixture a little, then puree in a blender, taking care not to burn yourself (or tarnish things with the blueberry juice, which will stain). Taste and add more sugar or cinnamon if necessary. Chill, then stir in yogurt or sour cream. Serve cold, garnished with more yogurt or sour cream.