Combine the blueberries and sugar in a small saucepan. Keep on low flame, stirring occassionally till the blueberries become mushy. Add the vanilla, switch off and allow it to cool completely.
In a bowl, whip the cream till stiff. Add the condensed milk and the blueberry compote and fold in.
Divide into four serving bowls and keep into the fridge for the mousse to set, around two hours.
Serve cold.