In a small saucepan, bring 1¾ cups (259 grams) blueberries, sugar, lemon juice, and salt to a boil over medium-high heat, stirring frequently until sugar dissolves. Reduce heat to medium; cook, stirring occasionally, until berries begin to soften and liquid begins to thicken, about 5 minutes.
In a small bowl, whisk together cornstarch and 1 tablespoon (15 grams) water until dissolved and smooth; stir into blueberry mixture. Cook over medium heat, stirring frequently, until mixture is thickened, about 1 minute. Remove from heat, and stir in remaining ½ cup (74 grams) blueberries. Let cool completely, stirring occasionally.
Notes
Blueberry Filling can be made a day ahead and refrigerated; before you use it, stir in a tablespoon of hot water to loosen it up.