Position a rack in the center of your oven and preheat to 375F. Line a 12-cup muffin tin with paper liners, or grease well with butter or cooking spray.

In a large bowl, whisk together the flour, baking soda, and salt until evenly combined. Set aside.

In a medium bowl, whisk the mashed bananas and sugar until mostly smooth. Add the melted butter, eggs, vanilla, and yogurt. Whisk until fully combined and glossy.

Pour the wet mixture into the dry ingredients. Use a rubber spatula to fold together until just combined - stop when you still see a few streaks of flour. Overmixing makes tough muffins.

Gently fold in the blackberries, being careful not to crush them. The batter will look slightly streaky purple - that's fine. If berries are very large, halve them first.

Divide batter among the 12 muffin cups, filling each about three-quarters full. Bake 20 to 22 minutes, until the tops are domed and a toothpick inserted in the center comes out clean or with just a few moist crumbs.

Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely. Serve warm or at room temperature.