Preheat oven to 350°F. Prepare a baking dish – between 8 and 10-inches round — with pan spray. In a mixing bowl, sift together flour, baking powder, and salt.
Using an electric mixer, cream together butter and 1 cup of sugar in another bowl. Combine the butter mixture, flour mixture, and milk on medium speed until completely incorporated – approximately 1 minute. Spread batter into the bottom of prepared pan.
In a medium bowl, mix the blackberries, remaining 3⁄4 cup sugar, and fruit juice. Pour the blackberry mixture on top of the batter.
Place pan in oven and bake until crust is just turning golden brown – approximately 45 minutes. Let cool for 5-10 minutes and serve with ice cream.