Preheat the oven to 350 degrees F and spray a muffin tin with nonstick spray or use paper liners.
Rinse the frozen berries and let them drain in a sieve.
In a mixing bowl, whisk the flour, baking powder, and salt to evenly distribute. In another bowl, beat the egg, then whisk in the sugar, and sour cream. Whisk until the mixture is throughly combined then add in the buttermilk and melted butter.
Scatter the blackberries in the dry ingredients and then add the wet ingredients. Gently fold to combine, there should not be large pockets of flour but streaks of flour are okay. Do not overmix the batter.
Divide the batter into your muffin tin with a large spoon or ice cream scoop and bake until a toothpick or skewer inserted into the center of the muffins comes out clean, about 20 - 25 minutes. Remove them from the pan and cool on a rack.