Rinse and gently dry 2 cups fresh blueberries. Divide the blueberries into two equal portions.
Add 1 cup blueberries, ½ cup granulated sugar, ½ cup water, and 1 teaspoon lemon juice to a small saucepan.
Cook over medium-low heat, stirring occasionally, until the mixture comes to a gentle boil and the blueberries begin to break down.
In a small bowl, whisk together 2 tablespoons cornstarch and 2 tablespoons water until smooth.
Slowly whisk the cornstarch mixture into the simmering blueberry mixture.
Continue cooking for 3–5 minutes, stirring constantly, until the sauce thickens.
Remove the saucepan from the heat and gently fold in the remaining 1 cup fresh blueberries.
Allow the blueberry sauce to cool completely.
Transfer to an airtight container and refrigerate until ready to use.