Step 1

Puree the blueberries and sugar in a blender until smooth. Pour through a fine-mesh sieve set over a bowl; press on the solids to extract as much liquid as possible. Discard solids. Stir in the buttermilk, lemon zest and vanilla extract.

Step 2

Churn the blueberry mixture in an ice cream maker according to the manufacturer's instructions, then transfer to an airtight container. Freeze until hardened, at least 2 hours. Sherbet will keep in the freezer for up to 2 weeks.