Heat the oven to 350 degrees F. Grease an 8-by-8-inch baking pan with nonstick cooking spray. Puree the undrained black beans and their liquid in a food processor until smooth. Put the brownie mix and pureed beans in a large bowl and beat with an electric mixer until combined, about 2 minutes. Transfer the batter to the prepared pan. Bake for 25 to 30 minutes, or until a toothpick inserted near the center comes out clean. Let cool on a wire rack and cut into squares.