In a small dish, whisk together 2 tablespoons water and the cornstarch.
In a medium saucepan over medium-high heat, combine the remaining 1 cup water, sugar, maple syrup, vanilla, salt, and blueberries and stir until the sugar dissolves. Bring to a boil, stirring occasionally. Reduce the heat and gently simmer until slightly thickened, about 10 minutes for fresh berries or about 14 minutes for thawed frozen berries.
Add the reserved cornstarch mixture to the pan and continue to cook, stirring constantly and crushing blueberries with the back of a spoon or with a potato masher, until the sauce thickens to your desired consistency, 2 to 7 minutes. Remove the syrup from the heat and let cool for 5 minutes.
Strain the slightly cooled syrup through a strainer, if desired. (If you prefer a thicker, more sauce-like consistency, skip the straining. If you prefer a smoother, silkier syrup, definitely strain the syrup.) Serve the blueberry syrup immediately. Cover and refrigerate any leftover syrup for up to 1 week.