Heat oven to 375 degrees Fahrenheit/200 degrees Celsius. Grease a 9- or 8-inch/21-centimeter loaf pan with butter, line it with a parchment paper sling and butter the paper. Set the pan aside.
In a separate bowl, whisk together flour, baking powder, salt and almond flour. Set aside.
Place butter, sugar, lemon zest and vanilla extract in the bowl of a stand mixer fitted with the paddle attachment. Beat on high speed for 3 to 4 minutes, until light, then lower speed to medium. Add eggs in three additions, scraping down the sides of the bowl a few times as necessary. The mix may split (look curdled) a little but don’t worry: It’ll come back together once you add the dry ingredients.
With the stand mixer on low, add the reserved dry ingredient mixture in three additions, mixing just until no dry specks remain. Fold in about 3/4 of the blueberries by hand, then scoop batter into the prepared loaf pan.
Bake for 15 minutes, then sprinkle the remaining blueberries over the top of the cake. Check the cake at the 10 minuteReturn to the oven for another 15 to 20 minutes, until cake is golden brown but still uncooked. Cover loosely with foil and continue to cook for another 25 to 30 minutes (less for a 9-inch pan, more for an 8-inch pan), or until risen and cooked, and a knife inserted into the middle of the cake comes out clean.
Remove from oven and set aside in its pan to cool for 10 minutes before removing cake from pan and placing on a wire rack to cool completely.
When cake is cool, make the icing: Add lemon juice and icing sugar to a bowl and whisk together until smooth, adding a bit more juice if necessary, just until the icing moves when you tilt the bowl. Pour over the cake and gently spread out. The blueberries on the top of the cake may bleed into the icing a little, but this will add to the look.
Let icing set (about 30 minutes), slice and serve.