Preheat the oven to 375°F. Grease a 12-well muffin tin and set aside.
Whisk together the flour, sugar, baking powder, and salt in a large bowl.
Once those are mixed, add in the lemon zest and whisk throughout.
Combine the yogurt, melted butter, lemon juice and egg. Whisk together rapidly to ensure a complete mix. Add to the dry ingredients and combine.
Fill a muffin well 1/3 of the way, then place a blackberry on the batter. Place another dollop of batter, then another blackberry. Fill the rest of the tin to 2/3 full of batter. Roll 3 blackberries in sugar and place on top of the muffin. Sprinkle on extra lemon zest if desired. Repeat for all 12 muffins.
Bake at 375 for 14-17 minutes, until the tops spring back when touched and a toothpick inserted comes out clean. Remove and cool on a wire baking rack.
Store in a closed container at room temperature for up to 3 days, or freeze for up to 3 months.
Notes
By adding the berries into the muffin tin, you ensure that you get berries in each muffin.