In a saucepan, combine the sugar, flour and salt. Gradually add cream and lemon zest until blended. Bring to a boil; cook and stir for 2 minutes or until thickened and bubbly. Remove from the heat. Stir a small amount of hot mixture into egg yolks. Return all to pan; bring to a gentle boil, stirring constantly. Remove from the heat; stir in butter and vanilla. Pour into four parfait glasses or dessert dishes. Cool.
Drain blueberries, reserving juice. Spoon blueberries over custard. In a saucepan, combine cornstarch and blueberry juice until smooth. Bring to a boil over medium heat; cook and stir for 1-2 minutes or until thickened. Spoon over berries.