Process blackberries in a food processor or blender until smooth, stopping to scrape down sides. Press blackberries puree through a fine wire-mesh strainer into a large bowl, discarding solids. Add sugar, buttermilk, and 1 teaspoon vanilla extract to bowl, and stir until well blended.
Pour blackberry mixture into freezer container of a 4-quart ice-cream maker, and freeze according to manufacturer's instructions. Garnish each serving, if desired.
