In the jar or bowl of a blender or food processor, add the Greek yogurt, sugar, blueberries and acai, in that order, and blend until smooth.
Pass the mixture through a fine sieve or chinois strainer to remove the seeds and grit. Definitely take the time to sieve, or the frozen yogurt will have a gritty texture.
Whisk in the limoncello and lemon juice.
Chill the frozen yogurt base for at least 1 hour in the refrigerator.
Put mixture into an ice cream maker and churn for 20 to 30 minutes.
Transfer to and store in an airtight container.
