In a saucepan over high heat, bring 4 cups blueberries and the stout to a boil, reduce heat and simmer for 20 minutes. Add the vanilla and set aside to cool completely.
Preheat oven to 350°F with oven rack positioned in the centre. Generously butter a bundt pan and dust evenly with a tablespoon of cocoa.
Toss remaining blueberries with 1 tsp of flour and set aside, do not thaw if using frozen.
In a bowl, whisk the flour, cocoa, baking powder, baking soda, cinnamon and salt, set aside.
In another extra large bowl, using a hand mixer, cream the butter and sugar then add the eggs one at a time and beat until smooth.
On low speed, mix in the dry ingredients alternately with the sour cream and stout mixture.
Gently fold in the floured blueberries.
Pour batter into prepared bundt pan and bake for 90 minutes until toothpick inserted in the centre comes out clean.
Allow to cool then invert onto a wire rack or plate.
In a saucepan, simmer the blueberries, sugar and water on low heat for 10 minutes, until reduced. Pass mixture through a fine mesh sieve into a bowl, reserve syrup and discard the pulp.
Return the syrup to the same saucepan on low heat and stir in chocolate, cream and vanilla until melted and smooth. Cool slightly and pour immediately onto cooled cake. Serve with custard or crème anglaise.