Bring first 5 ingredients to a boil in a saucepan over medium-high heat, stirring often. Remove from heat, and slightly cool.
Process blueberry mixture and lemon juice in a blender or food processor until smooth, stopping to scrape down sides. Cover and chill until ready to serve.
Stir in half-and-half just before serving. Serve in stemmed glasses. Garnish, if desired.