In a heavy saucepan, over medium heat, melt the berries and juice, stirring to break up the frozen lumps. Turn up the heat and bring to full rolling boil. Remove from heat. In a bowl, strain fruit and juice by pressing on pulp, removing the seeds. There should be 1½ cups of juice. Rinse the sauce pan, add the juice and sugar, bring to boil again. Mix the arrowroot with cold water and add juice. Reduce heat and cook only a few seconds, until thickened. Pour sauce into bowl. Chill. Add Chambord if desired.
