Butter two 8 x 4 inch loaf pans and line the bottom and two long sides of the pans with parchment paper. I use one sheet and let it overhang a bit, then use the parchment to lift the loaf out of the pan when ready.
In a large bowl, beat together the butter, sugar, eggs and vanilla extract until mixed.
In a separate bowl, combine the flour, baking powder and salt and stir with a fork.
Add the flour mixture to the egg mixture, alternating with the milk. Mix just until combined.
In the bowl that you mixed the flour in, add the blueberries, lemon zest and 4 tablespoons of flour, tossing gently until coated. The flour will help prevent the berries from sinking to the bottom.
Carefully and quickly fold the berries into the batter, then divide the batter equally between the prepared pans.
Bake in preheated oven for between 45 - 55 minutes, or until a cake taster inserted into the centre of the loafves comes out clean.
Cool the loaves in the pan for 15 minutes, then remove from the pan using the parchment paper handles and let cool on a rack.
Combine the juice and icing sugar in a bowl and beat with a mixer until smooth. Drizzle the top of the cooled loaf with the glaze, letting it pour down the sides.