Combine the blueberries, honey, and lemon juice in a medium saucepan. (The exact amount of honey depends on how tart your blueberries and how sweet your sweet tooth.) Place over medium-low heat and cook until the blueberries soften and the syrup that forms starts to thicken, 10 to 20 minutes (if you're using frozen blueberries, it may take a little longer).
Remove the pan from the heat, stir in the salt, and let cool to room temperature, about 45 minutes.
Dump the mixture into a blender, add the yogurt, and blend until fully incorporated. Taste and add more honey if needed, keeping in mind that your frozen yogurt will taste less sweet once frozen (cold mutes flavors). Place the blueberry frozen yogurt mixture in a resealable container with a lid, cover, and refrigerate until cold, at least 2 hours and up to 24 hours.
Process the mixture in an ice cream maker according to manufacturer's instructions. Spoon into an airtight container and freeze for at least 2 hours to allow the texture to firm slightly before serving. (The frozen yogurt is best served within hours of churning but rest assured it will keep in the freezer for up to several days without turning into an impenetrable chunk of purple ice.)