In a food processor or blender, puree the fresh or frozen blackberries.
11 ounces fresh or frozen blackberries
Strain the puree through a fine mesh sieve. Use a wire whisk to stir the puree and push it through the sieve. Set aside puree for use in the cupcakes and in the frosting.
To make the cupcakes:
Preheat the oven to 350°F. Line a cupcake pan with paper liners.
In a small bowl, stir together flour, baking powder, and salt. Set aside.
1 cup flour, 2 teaspoons baking powder, 1 teaspoon salt
In a separate bowl or liquid measuring cup, whisk together the milk and blackberry puree. Set aside.
½ cup milk, ½ cup blackberry puree
In the bowl of a stand mixer, cream together the butter and sugars with an electric mixer on medium-high speed until light and fluffy.
6 tablespoons butter, 1 cup sugar
Add the eggs one at a time, scraping down the bowl between additions. Add the vanilla extract.
2 eggs, 1 teaspoon vanilla extract
Add half of the dry ingredients to the mixer bowl and blend on low speed until just combined. Add the blackberry mixture, and mix on low until just combined. Mix in the remaining dry ingredients on low speed until just incorporated. Do not overmix.
Divide the batter evenly between the cupcakes.
Bake 12-15 minutes or until a toothpick inserted in the center comes out clean. Allow to cool in the pan 10-15 minutes. Transfer to a wire rack and allow to cool completely.
To make the frosting:
Add the cream cheese to the bowl of a stand mixer fitted with the whisk attachment. Beat the cream cheese until smooth.
8 ounces cream cheese
Add the powdered sugar and blackberry puree in a large mixing bowl. With an electric mixer, mix on medium speed until smooth and well blended.