Combine 1/4 cup of the blueberries, 2 tablespoons water, 1 tablespoon honey and the thyme sprigs in a small heavy saucepan. Coarsely mash the blueberries using a fork or a potato masher. Place over medium-high heat and bring to a boil. Remove the pan from the heat, cover and let the mixture stand for 15 minutes. Uncover the pan and let the syrup cool slightly. Discard the thyme sprigs.

Place the remaining blueberries, remaining 1/3 cup honey, yogurt, agave, lemon juice and cooled blueberry syrup in a blender or food processor. Blend until the mixture is almost smooth.

Transfer the yogurt mixture to a glass container with a tight-fitting lid and freeze until firm, at least 8 hours or overnight (the frozen yogurt will still be slightly soft).

Scoop the frozen yogurt into bowls and serve.