Combine flour and next 4 ingredients in a large bowl, whisking to combine. Stir in oats.
Beat shortening and brown sugar together until incorporated. Beat in eggs until mixture is fluffy; gradually stir in buttermilk and 1/4 teaspoon lemon zest. Add the flour mixture to the wet ingredients, and stir just until all dry ingredients are moistened. Stir in pecans. Gently fold blueberries into the batter by hand.
Spread batter into a greased and floured 9x5x3-inch loaf pan. Bake for 55-60 minutes, or until wooden toothpick inserted in the center comes out clean. Cool in pan on wire rack for 10 minutes before turning out. Spread immediately with the glaze, and allow to cool for at least 30 minutes before slicing.
For the glaze
Place powdered sugar, lemon juice, milk and salt in a medium bowl, and whisk to combine, adding more milk as needed, 1 teaspoon at a time, until the mixture is of thick but pourable consistency. Drizzle over the prepared loaf.
Notes
When using frozen blueberries, there is no need to thaw them before incorporating them into the batter.