Blackberries give this summer drink (left) its gorgeous deep-purple hue. To turn it into a Fizzy Purple Passion, simply add some seltzer.
Chill the berries on a plate in the freezer until firm (not frozen solid), about 35 minutes.
In a blender, puree the chilled berries with the ice cube and milk. Strain the mixture into a bowl to remove the seeds. Rinse the blender and pour the mixture back in. Add the sugar, the vanilla extract, and 3 to 4 big scoops of ice cream or frozen yogurt and blend the mixture until smooth, adding more milk if it's too thick.
Divide the shake among three chilled glasses. If you like, stir a few tablespoons of seltzer into each for a little fizz. Garnish the tops with whole berries and serve. Makes three 8-ounce servings.