Preheat the oven to 180°C and grease a 26cm x 20cm rectangular baking tin.
Cream the butter and sugar in a bowl until light and fluffy.
Beat in the eggs one at a time until the mixture is well combined.
Stir in the soured cream, lemon juice, zest, flour, and baking powder.
Fold the blueberries into the batter and transfer to the prepared tin.
Bake for 40-45 minutes until a skewer inserted comes out clean.
Allow the cake to cool completely before removing from the tin.
Whisk the butter and icing sugar together for the topping until fluffy.
Mix in the cream cheese and lemon juice until smooth.
Spread the frosting over the cake and garnish with blueberries and icing sugar.