1/2 cup fresh lemon juice, plus 1 tablespoon grated lemon zest
One 1/4-ounce package unflavored powdered gelatin
2/3 cup plus 6 tablespoons sugar
1/4 cup light corn syrup
2 cups plain whole-milk yogurt
1/2 cup heavy cream
3/4 cup blueberries
2 teaspoons water
Fill a large bowl with ice water. Pour 2 tablespoons of the lemon juice into a small bowl. Sprinkle the gelatin over the lemon juice and let stand for 5 minutes.
Meanwhile, in a small saucepan, whisk the remaining 6 tablespoons of lemon juice with 2/3 cup of the sugar and the corn syrup. Bring to a boil and cook over moderate heat until the sugar dissolves, 1 minute. Remove from the heat and stir in the lemon gelatin.
In a medium bowl, mix the yogurt with the zest. Stir in the lemon juice mixture, then whisk in the cream. Set the yogurt base in the ice water bath and let stand, stirring occasionally, until cold, 20 minutes.
Meanwhile, in a saucepan, mix the blueberries with the remaining 6 tablespoons of sugar and the water. Simmer over moderate heat, until saucy, 4 minutes. Let cool.
Pour the lemon yogurt into an ice cream maker and freeze according to the manufacturer’s instructions.
Scoop alternating spoonfuls of the yogurt and blueberry sauce into a plastic container. Press a sheet of plastic wrap directly onto the surface and close with an airtight lid. Freeze until firm, about 4 hours.