In a dry skillet, toast walnuts over medium-high heat for 1 to 2 minutes.
In a large bowl, combine all-purpose flour, baking powder, baking soda and salt. Whisk to combine.
In a medium bowl, combine sugar, lemon zest, buttermilk, eggs, vanilla, melted butter and oil. Whisk just to combine.
Make a well in center of dry ingredients. Add liquid ingredients and mix gently just to combine; do not over-mix. Gently fold in walnuts and blueberries.
Pour batter into a greased and floured 8 x 4-inch loaf pan. Bake in preheated oven for 55 to 60 minutes or until tester inserted in center comes out clean. Cool on rack for 20 minutes; remove from pan, cool to room temperature.
To make the glaze, combine sugar and lemon juice in a small saucepan. Bring to boil over high heat, then reduce heat and continue stirring until sugar dissolves.