Heat oven to 400°F. Line a 12-cup muffin pan and lightly mist the liners.
Whisk flour, cocoa, sugar, baking powder, baking soda, salt, and espresso powder in a large bowl.
In another bowl, whisk eggs, buttermilk, oil, and vanilla until smooth.
Pour wet ingredients into dry. Stir gently until just combined; the batter should look thick.
Fold in chocolate chips and about 1 cup of the blackberries, keeping chunks whole.
Scoop batter into liners, about 3/4 full. Top with remaining berries and sprinkle coarse sugar.
Bake 16–18 minutes, rotating once, until a toothpick comes out with a few moist crumbs.
Cool 5 minutes in the pan, then transfer muffins to a rack to finish cooling.