If you can't find black raspberries, feel free to substitute red ones. Martha made this recipe on episode 702 of Martha Bakes.
Make the cake: Preheat the oven to 350 degrees. Butter a 9-inch square pan and line with parchment paper, leaving a 2-inch overhang. Butter parchment; set pan aside. Combine milk and lemon juice; let stand 15 minutes.
In a medium bowl, whisk together 2 cups flour, baking powder, and salt; set aside. In the bowl of an electric mixer fitted with the paddle attachment, beat butter, zest, and sugar until light and fluffy. Add egg and beat to combine. Add the flour mixture in three additions, alternating with the milk mixture and beginning and ending with the flour.
Place berries in a bowl and gently toss with remaining tablespoon flour. Fold half of the berries into the batter, then transfer batter to prepared pan. Evenly distribute remaining berries over the surface of the batter.
Make the crumb topping: In a medium bowl, whisk together flour, sugar, cinnamon, and salt; using your hands, work in butter.
Sprinkle crumb topping over berries, using your fingers to clump some of the topping together to form large crumbs. Transfer to oven and bake until top of cake is golden and a cake tester inserted in the center comes out clean, 35 to 40 minutes.
Let cool in pan on a wire rack for about 30 minutes. Loosen cake from the sides of the pan with an offset spatula or a knife. Using parchment overhang, lift cake out of pan and transfer to a rack to cool completely (still on paper). On a cutting board, use a serrated knife to cut into squares. Cake may be stored in an airtight container at room temperature for up to 3 days.