In a small saucepan, combine cornstarch with sugar; whisk in lemon rind, juice and water. Bring to boil over medium heat, stirring constantly, and cook for 2 minutes or until thickened and smooth. Blend a little of the hot mixture into egg yolk; stir yolk mixture into saucepan. Cook over low heat, stirring constantly for 2 minutes. Remove from heat and stir in margarine. Let cool, stirring frequently to prevent skin from forming on top. Cake: Line 1 5 x 1O inch jelly-roll pan or baking sheet with foil; thoroughly grease and flour foil. In large bowl, beat egg whites, salt and cream of tartar just until mixture mounds on spoon (not quite to soft peak stage). Using spatula, fold in granulated sugar, a large spoonful at a time. Sift half of the flour over egg-white mixture and fold in gently; repeat with remaining flour. Fold in lemon juice, vanilla and almond extract. Spread in prepared pan. Bake at 300 degrees for 25 minutes or until firm to the touch. (Cake will be light in color.) Stir half of icing sugar over cake; cover with tea towel then inverted baking sheet. Turn cake over and carefully remove jelly-roll pan and foil. Trim any crusty edges. While cake is hot, roll up in towel, starting at long side, jelly-roll fashion; let cool. (Cake and filling can be prepared to this point, covered and refrigerated for up to 1 day.) Unroll cake and spread evenly with lemon filling. Roll up cake using towel to help roll. Sift remaining icing sugar over top. Place seam-side-down on serving platter. Just before serving arrange blueberries around lemon roll. Makes 8 servings.