Our Test Kitchens awarded its highest mark to this simply sensational dessert.
Stir together 4 cups blackberries and sugar in a bowl; let mixture stand 30 minutes.
Process blackberry mixture in a food processor or blender until smooth, stopping to scrape down sides. Pour through a fine wire-mesh strainer into a 9-inch square pan, discarding solids; stir in buttermilk. Cover and freeze 8 hours.
Break frozen mixture into chunks, and place in a bowl; beat at medium speed with an electric mixer until smooth. Return to pan; cover and freeze 3 hours or until firm. Garnish, if desired.
* 2 (14-ounce) packages frozen blackberries, thawed, may be substituted for fresh blackberries.
Kitchen Express: Double the ingredient amounts and combine blackberry juice and buttermilk in freezer container of a 4-quart electric freezer. Freeze according to the manufacturer's instructions. Pack freezer with additional ice and rock salt, and let stand 1 hour before serving.