Begin by preheating the oven to 375°F/200°C. Next, generously grease a 9- or 8-inch/21-cm loaf pan with butter, ensuring you also line it with parchment paper. Don't forget to butter the paper. Now, set the pan aside.
In the bowl of a stand mixer with the paddle attachment, place butter, sugar, lemon zest, and vanilla extract. Start by beating on high for 3-4 minutes until it becomes light, then switch to medium speed. Gradually incorporate the eggs in three portions, making sure to scrape the sides of the bowl as needed. Don't worry if the mixture temporarily separates; it will come back together when you introduce the dry ingredients.
To prepare, mix flour, baking powder, salt, and almond flour in a separate bowl. Gradually add the dry ingredients in three stages using the stand mixer on low speed until no white specks are visible. Next, gently incorporate approximately 75% of the blueberries by hand before transferring the batter to the prepared loaf pan.
Bake for 15 minutes, add remaining blueberries, then bake for 15-20 more until golden and cooked. Cover with foil and bake 25-30 more (less for 9-inch, more for 8-inch), until a knife comes out clean. Cool in the pan for 10 minutes, then transfer to a wire rack to cool.
Once the cake cools, prepare the icing by whisking lemon juice and icing sugar in a bowl until smooth. Adjust juice for desired consistency. Pour over the cake and let it set for 30 minutes before slicing and serving.