Preheat oven to 180°C. Line loaf tin.
Combine flour, baking powder and salt in a bowl. In a large, separate bowl, rub together with your fingers, sugar and lemon zest. Beat in softened butter until fluffy. Add in eggs, 1 at a time and beat. Beat in 2 tbsp of the lemon juice. Stir in flour mixture and buttermilk in parts.
Add most of the blueberries into the batter and mix well. Pour batter into the prepared tin and sprinkle the remaining blueberries on top. Bake for 65 to 70 minutes or until the cake springs back when tapped on top. Let cool completely.
In bowl, whisk icing sugar with remaining 1 tbsp lemon juice until smooth; whisk in a few drops of water until the icing becomes the consistency of honey. Drizzle over top of loaf; let stand until set. Top with more blueberries and lemon zest if needed.