Place the sugar, cornstarch, cinnamon, salt, and water in a medium sized saucepan and stir until smooth. Gently stir in the blueberries and place the saucepan over medium heat. Cook (simmer) the sauce until the liquid thickens and becomes clear, stirring occassionally (about 10 minutes). (Some of the blueberries will break down but others will remain whole.) (The longer you cook the sauce the thicker it will become.)
Remove from heat and stir in the vanilla extract, if using, and the lemon zest and juice. Taste and add more lemon juice if needed. Let cool and then cover and refrigerate for up to a week.