In a saucepan, bring sugar, 1 cup water, and lavender, if using, to a boil. Add salt and simmer 2 minutes. Transfer sugar syrup to a metal bowl and let cool to room temperature. Strain and discard lavender.
In a blender, purée sugar syrup, blueberries, and lemon juice. Strain and discard seeds and skin. Cover and chill sorbet mixture at least 30 minutes and up to overnight.
Freeze in an ice cream maker according to manufacturer's instructions. Transfer to a freezer-safe dish, cover, and freeze until hard, at least 1 hour and up to overnight.
Note: Nutritional analysis is per serving.