Mix the ingredients: Add the frozen blueberries, granulated sugar, and water to a saucepan. Stir to combine, and bring to a boil over medium heat. Reduce heat to low to establish a simmer.
Simmer to thicken: Simmer for 10-15 minutes until thickened to a syrupy consistency that lightly coats the back of your spoon. About halfway through, use a fork or potato masher to mash the blueberries. Stir in vanilla extract and a pinch of salt and remove from heat.
For completely smooth syrup, pour the mixture through a fine mesh sieve, pressing out as much of the juice as possible.For a jammy consistency (my personal preference), lightly blend (with a regular blender or immersion blender) to your desired texture. (Just make sure any blueberry skins are small enough to pass through your straw!) Transfer your blueberry syrup to a sealed container and keep it in the fridge for up to a week.