Preheat oven to 350° F, 177C, or gas mark 4. Spray a 8 ½ x 4 ½ loaf pan with cooking spray and set aside.
In a small bowl mix the granulated sugar, brown sugar, and lemon zest together. Use a spatula to press the sugar against the sides or use your fingers to rub the sugar and zest together. The sugar will start to smell like lemon.
In a mixing bowl, whisk together the flour, salt, and baking powder. Once combined add the the sugar and lemon mixture.
In the smaller bowl, combine the buttermilk, oil, eggs, lemon juice and vanilla extract. Whisk until evenly mixed. Pour the wet ingredients over the dry and gently mix until everything has combined. Gently fold in the blueberries and pour into the loaf pan.
Bake for 70-80 minutes, or until toothpick or knife inserted into the center comes out clean. You may need to cover the cake with foil halfway through baking if the top is starting to brown too quickly.
Let the cake cool in the baking dish on a cooling rake for 15 minutes before gently removing and cooling completely on a cooling rack.
Making the glaze
While the cake is cooling, in a small bowl, whisk together powdered sugar and lemon juice. Drizzle the glaze of the cooled loaf cake. The cake needs to be cool before adding the icing.