Simmer: Add the blueberries, water and sugar to a small saucepan over medium heat. Cook, stirring occasionally, until the mixture boils and the blueberries break down, about 5 minutes.
Reduce: Stir in the maple syrup and salt. Continue cooking to reduce and thicken, about 5 minutes.
Cool: Remove from heat and let cool slightly. The syrup will continue to thicken as it cools.
Notes
Swap in blackberries or raspberries for a different berry syrup, or use honey or agave in place of maple syrup for a different depth of sweetness.