Add all ingredients to a small saucepan. Simmer over medium-low heat for 12 minutes, stirring occasionally. Mixture should be bubbling slightly.
Remove mixture from heat and let cool for at least 10 minutes. Mixture will thicken as it cools.
Notes
Around the 7 minute mark, the berries should start to pop and the liquid should be getting thick and turning purple. If that's not happening, turn the heat up just a bit for a minute or two.
If you use frozen blueberries you may need to either turn up the heat a bit at the beginning or increase the cooking time by a few minutes.