Preheat your oven to 350° F. Position the rack to the middle of the oven. Spray or grease a 4.5 x 8.5" loaf pan and set aside.
Cream the butter and sugar in a large mixing bowl. Add the egg, vanilla, lemon juice, and lemon zest, mixing well.
½ cup butter, 1 cup sugar, 1 egg, 1 teaspoon vanilla, 2 tablespoon lemon juice, 1 tablespoon lemon zest
In another mixing bowl, combine the flour (except for the tablespoon you set aside to toss the berries), baking powder, baking soda, and the salt. Mix well.
1½ cup cups flour, 1 teaspoon baking powder, ½ teaspoon baking soda, ¼ teaspoon salt
Add half the dry ingredients to the bowl of wet ingredients, mixing well with the hand mixer. Add the sour cream and mix in.
½ cup sour cream
Add the remaining dry ingredients and blend well. Toss the berries with the reserved tablespoon of flour.
1 cup fresh blackberries
Transfer about 1 to 1½ cups of the batter to the bowl of berries and gently fold in using a spatula.
Pour half the plain batter to the loaf pan, spreading out evenly. Then, spread the batter with the berries in the pan. Last, spread the remaining plain batter on top. Bake for 45 minutes in the preheated oven. When done, remove and cool on a rack, tipping out after 5 minutes.