In a small bowl, combine zucchini, buttermilk, lemon zest & lemon juice; toss to combine. In a large bowl, cream butter & sugar until light & fluffy. Beat in eggs, one at a time. In a third bowl, whisk 3 1/4 cups flour, baking soda & salt; gradually add to creamed mixture alternately with zucchini mixture, mixing well after each addition. Toss blueberries in remaining flour; fold into batter.