Combine 1 1/2 cups blueberries with the sugar, lemon juice and water in a large heavy saucepan. Cook over medium-low heat, stirring constantly, until the berries pop and a sauce forms, 5 to 10 minutes.Advertisement
Remove the pan from the heat and stir in framboise (or Chambord), if using. Transfer the sauce to a bowl, cover and chill until cold.
Just before serving, stir in the remaining blueberries and raspberries.