Place the blueberries, sugar, and 1/4 cup water in a saucepan and heat over medium heat. Bring to a low boil, then cook, stirring occasionally, for about 5 minutes until the blueberries begin to break down, mashing them with a wooden spoon. Make sure not to let the pot boil over.
Place the cornstarch in a small bowl and mix with 2 tablespoons water. Add it to the sauce and simmer for about 1 minute, stirring constantly, until thickened.
Remove from the heat and stir in the lemon juice and salt. The sauce will thicken even more as it cools. Serve immediately or refrigerate until serving (store up to 1 week refrigerated).