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.
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.