Bring the stock to a boil. Meanwhile, melt the butter in another pan, add the onion and cook over low heat, stirring occasionally, for 5 minutes until softened. Add the rice and cook, stirring constantly, until the grains are coated in butter. Sprinkle in the wine and cook until it has evaporated.
Set aside 2 Tbsp of the blueberries and add the remained to the pan. AAdd a ladleful of the hot stock and cook, stirring, until it ahs been absorbed. Continue adding the stock, a ladleful at a time, and stirrung until each addition has been absorbed. This will take 18-20 minutes. When the rice is tender, stir in the cream and transfer to a warm serving dish. Garnish with the reserved blueberries and serve with Parmesan.