In a medium saucepan, cook the blueberries and water over medium heat for 15 minutes, until the liquid has reduced by half.
Strain the mixture over a measuring cup, discarding the solids and reserving the liquid. Note the amount of liquid, then return it to the warm saucepan.
While the liquid is still hot, stir in an equal amount of sugar to create a 1:1 syrup.
Transfer to a food-safe airtight container, such as a mason jar, and store in the refrigerator.
Notes
After reducing the berries, you should have about 4 ounces of liquid. To make a syrup, use sugar in proportion to the amount of liquid to control the sweetness and consistency.
For a 1:1 simple syrup, measure the reduced liquid and add an equal amount of sugar (about ½ cup).
For a 2:1 rich syrup, use double the amount of sugar (about 1 cup). Be sure to strain out the solids before adding the sugar to ensure a smooth syrup.