Place the chopped chocolate in a small heatproof plastic bag. Set the bag in a mug or heatproof glass and pour boiling water around it, keeping all water out of the bag. Let stand for 3 minutes, then dry the bag thoroughly and snip off a small corner.
Pipe the melted chocolate into a silicone mold, tilting and rotating to coat the sides evenly. Transfer the mold to the freezer until the chocolate is fully set, about 10 minutes.
Meanwhile, stir the yogurt and blueberries together until evenly combined. Spoon the mixture into the chilled mold.
Pipe the remaining chocolate over the top, smoothing gently if needed. Freeze until firm and fully set, at least 2 hours.
Unmold and serve straight from the freezer. An extra drizzle of chocolate on top is optional and purely decorative.