Preheat the oven to 180C/350F/Gas 4. Grease and line a rectangular baking tin (approximately 26cm x 20cm/10½in x 8in).
Beat the butter and sugar together in a bowl until light and fluffy, then beat in the eggs, one at a time, until well combined. Stir in the soured cream, lemon juice and zest, flour and baking powder until well combined, then carefully fold in the blueberries.
Spoon the mixture into the cake tin and bake for 40-45 minutes, or until a skewer inserted into the cake comes out clean. Remove the cake from the oven, and set aside until completely cool.
Meanwhile for the cream cheese topping, beat the butter and icing sugar together in a bowl until light and fluffy. Beat in the cream cheese and lemon juice, then spread over the top of the cake. Sprinkle over the blueberries and dust with icing sugar.