Stir together 4 cups of fresh blackberries and sugar in a bowl and let stand for 30 minutes.
Process blackberry mixture in a food processor or blender until smooth, stopping to scrape down the sides. Pour through a wire-mesh strainer into a 9 inch square pan, discarding solids; stir in buttermilk. Cover and freeze for 8 hours.
Break frozen mixture into chunks and place in bowel; beat with electric mixer until smooth. Return to pan; cover and freeze again for 3 hours or until firm.
Garnish with whole black berries and/or blackberry syrup.
Yield: 1 quart taking about 15 minutes plus 30 min stand time and 11 hours freezer time.
Ice cream maker alternative - double the ingredients and combine the blackberry process mixture with the buttermilk in a 4-quart electric or hand operated freezer. Pack in ice and rock salt mixture and stir till contents stiffen significantly. Add more salt and ice and let stand for 1 hour without stirring.