Preheat the oven to 375°F and line a muffin pan with paper liners. Toss blackberries lightly with flour to keep them evenly distributed while baking.
Whisk together flour, sugar, baking powder, baking soda, salt, and cinnamon in a large bowl until fully combined and evenly mixed.
In another bowl, whisk eggs, melted butter, buttermilk, and vanilla extract until smooth and creamy without streaks.
Fold the wet mixture into the dry ingredients gently until just combined, then carefully fold in the blackberries without crushing them.
Divide the batter evenly into muffin cups, filling each about three-quarters full, then sprinkle with coarse sugar and lemon zest if desired.
Bake for 20 to 22 minutes until the tops are golden and a toothpick inserted into the center comes out mostly clean. Cool slightly before serving.