Cream butter and 1/2 cup sugar. Beat in egg, flour, vanilla and salt. Refrigerate 2 hours. Heat blueberries and 1 cup sugar in saucepan to boiling. Reduce heat; simmer uncovered, stirring frequently, 3 minutes. Cool. Heat oven to 375 degrees. Pat two-thirds of the dough into ungreased 13 x 9 x 2 inch baking pan, making 1/2 inch ridge around edge. Spread blueberry mixture evenly over dough. Roll out remaining third of dough on lightly floured surface to 9 x 4 inch rectangle. Cut into strips 1/2" wide and 9" long. Arrange strips in lattice pattern over filling. Bake until light brown, about 45 minutes. Cool on wire rack. Cut into bars.