How to Make It
Set aside 1 1/2 cups of the blackberries. Place the remaining 3 cups berries and 1/2 cup of water in a nonreactive 3 1/2 -quart saucepan. Cover, bring to a simmer over moderate heat and cook until the berries have softened slightly and exuded their juices, about 5 minutes.
Transfer the cooked berries to a strainer and press through to remove the seeds. Stir in 3 tablespoons of the sugar until dissolved; sweeten with more sugar if desired. Refrigerate for 1 hour. Stir in the kirsch.
Up to 2 hours before serving, peel, halve and pit the peaches. Working over a bowl, slice the peaches 1/8 to 1/4 inch thick. Sprinkle the remaining 3 tablespoons sugar over the peaches and gently toss.
Ladle the chilled blackberry soup into 6 soup plates, arrange the peach slices in the soup and add a scoop of vanilla ice cream and about 6 or 7 of the reserved blackberries to each serving. Serve at once.