Preheat the oven to 180°C/gas mark 4. Grease the edges of a 20cm or 23cm cake tin and dust lightly with flour.
Pour the contents of the yogurt pot into a small bowl and set aside.
In a separate mixing bowl, whisk together the eggs and caster sugar. Whisk thoroughly to obtain a light and fluffy mixture, or until a ribbon of a figure of 8 lies in the batter for a few seconds. Using an electric whisk will speed up the process.
Add the natural yogurt, then the oil and cornflour. Gently fold the sifted flour and baking powder into the mix. To finish, add the vanilla extract, grated chocolate and half the blueberries.
Pour the batter into the prepared tin and scatter some of the reserved blueberries on top, then dust with icing sugar. Bake in the oven for 30 to 35 minutes, until a skewer inserted into the centre comes out clean. Remove the cake from the tin and allow to cool on a wire rack.
Meanwhile, to make the syrup, bring the water to the boil in a small saucepan and add the sugar. Once the sugar has dissolved, add the grapefruit slices and the orange juice and zest. Reduce the heat to very low and simmer for 5 to 8 minutes, until the rind is translucent, then remove from the heat and allow to cool.
Drizzle a little citrus syrup over the cake and add some fresh blueberries to serve.