Preheat the oven to 375 degrees F. In a medium-size saucepan, melt the butter and unsweetened chocolate over medium heat until smooth. Remove from the heat and cool slightly. Stir in the sugar, egg, buttermilk, and vanilla. In a small bowl, combine the flour and baking soda. Gently mix with the liquid ingredients. Fold in the blueberries. Spoon the batter into well-greased muffin cups, filling to the top. Bake for 25 to 30 minutes, or until a toothpick inserted in the center comes out clean. Transfer the muffins to a wire rack to cool. Drizzle the cooled muffins with the semisweet chocolate.