1. Rinse the blueberries under cool water and transfer to a large non-reactive bowl

2. Crush the berries lightly with a potato masher to release their juices

3. Pour in the cold water and stir in the citric acid until dissolved

4. Cover the bowl tightly with a lid or plastic wrap and refrigerate for 48 hours, stirring once every 12 hours

5. Line a fine-mesh sieve with cheesecloth and place over a large pot

6. Pour the soaked mixture through the cloth, allowing it to drain fully—do not press to avoid cloudiness

7. Discard the solids or reserve the pulp for secondary use

8. Transfer the strained juice to a clean pot and stir in the sugar until completely dissolved

9. Heat the mixture over medium heat just until it reaches 85°C (185°F)—do not boil

10. Pour the hot syrup into sterilized glass bottles, leaving 2.5 cm (1 inch) headspace

11. Seal and cool to room temperature, then refrigerate for up to 4 weeks

12. To serve, dilute 1 part syrup with 3–4 parts cold water or sparkling water over ice

13. For the pulp byproduct, measure the reserved solids and transfer to a saucepan

14. Add 120 ml (½ cup) water per 150 g pulp and bring to a gentle boil

15. Simmer 5 minutes, then strain again and sweeten to taste if desired