Preheat the oven to Gas Mark 4, 180ºC, fan160ºC. Grease and line the bottom of a 20cm (8in) square tin with nonstick baking paper.
Break 175g (6oz) of the chocolate into a bowl and set over a pan of simmering water until melted and smooth. Cool slightly. Roughly chop the remaining chocolate into small pieces and set aside.
Using an electric mixer, whisk the butter and sugar together until fluffy. Whisk in the eggs and vanilla extract, then stir in the melted chocolate.
Sift the flour and 1/2 tsp salt over the mixture and fold in. Then gently fold in the chocolate pieces and blueberries.
Spoon mixture into prepared tin, spread into the corners and level the top with a plastic spatula. Cook for 35-40mins, until a cocktail stick inserted into the centre comes out with a small amount of sticky crumbs. Cool before cutting into 36 squares.