Fabulous Blackberry Sherbet
If the berries are full of tough seeds, remove seeds by forcing berries through a food mill or a mesh sieve into a large bowl. If berries are mostly seedless, place them in a large bowl. Stir in sugar and let stand at room temperature for 30 minutes. In the work bowl of a food processor fitted with the metal blade, puree berry mixture. Pour into a large glass or metal bowl.
Whisk in buttermilk. Cover and refrigerate until mixture is very cold, at least 4 hours, then stir well. Freeze sherbet in a gel-type ice-cream freezer according to the manufacturer's instructions. The sherbet will be soft, like a thick milk shake. To freeze hard enough to scoop, transfer to an airtight container, press plastic wrap directly onto the surface, and freeze until firm.