In a mixing bowl, cream together butter, sugar, apple sauce and apple juice. Beat in eggs, milk and vanilla. Combine flour, salt and baking powder; add to creamed mixture just until combined. Gently fold in blueberries. Pour into a 9x5x3 inch loaf pan. Bake at 350°F for 45 to 55 minutes or until a toothpick inserted near the center comes out clean. Cool for 10 minutes before removing from pan to a wire rack to cool completely.