How to Make It
Combine blackberries and sugar in a blender or food processor; process until smooth. Strain blackberry mixture through a fine sieve over a bowl; discard solids. Stir in buttermilk.
Pour mixture into the freezer can of an ice-cream freezer; freeze according to manufacturer's instructions. Spoon sherbet into a freezer-safe container; cover and freeze 4 hours or until firm.