In a large bowl beat butter and sugar until light and fluffy. Add eggs one at a time, beating well after each addition. Mix in lemon zest and vanilla. Combine the flour, baking powder and salt; add to creamed mixture alternately with lemonade, beating well. Fold in blueberries. Pour into a greased and floured tube pan. Bake at 350F for 45 to 50 minutes or until a toothpick comes out clean. Cool 20 minutes, then remove from pan, cool completely. In a small saucepan, combine cornstarch and sugar. Stir in water until smooth. Add blueberries and bring to a boil over medium heat, stirring constantly. Cook until thickened. Serve warm with cake.