10 minutes before the cake is ready prepare the syrup.
Combine all ingredients for the syrup in a small saucepan. Bring to the boil then simmer for about 6-8 minutes or until reduced and syrupy.
Remove cake from the oven and slowly pour hot syrup over hot cake. You may need to pour on half the syrup, let it soak in then pour over the remaining syrup otherwise the blueberries will bleed which doesn't make it taste bad but maybe just not as presentable.