Combine blueberries, sugar, 1/2 cup water, lemon zest, and salt in a medium saucepan.
Bring to a gentle simmer over medium heat and cook, stirring, until berries start to burst, 3–5 minutes.
Stir cornstarch with 2 tablespoons cold water in a small bowl to make a smooth slurry.
Stir lemon juice into the berries. Slowly pour in the slurry while stirring constantly.
Simmer 1–2 minutes until glossy and thickened to your liking.
Remove from heat and stir in vanilla. Cool; sauce thickens more as it cools.