In a bowl, combine cream, sugar, vanilla and rum flavoring. Whip until stiff. Fold in blueberries and ¼ cup of the crumbs. Pile mixture into demitasse cups. Sprinkle with remaining crumbs and decorate with additional blueberries. Freeze until hard, then wrap and store until needed. Serve frozen in demitasse cups set on saucers. Yield: 6 servings.