Combine water, sugar and blackberries in a small saucepan over medium heat. Stir until the sugar is dissolved.
Bring the mixture to a boil, stirring occasionally, then reduce heat to low. Let the mixture boil for 2-3 minutes, mashing the berries with a wooden spoon as you stir. You want to release as much of the berry juices as possible. Remove from heat and let cool.
Once cooled, strain the liquid into a glass jar or airtight container and discard the berry pulp. Use right away, or store in the refrigerator for up to one month.