To cut clean slices, dip a sharp knife into water before each cut.
Preheat the oven to 375 degrees. Coat a 13x9-inch baking pan with cooking spray.
Combine 2 cups of the flour, 3/4 cup of the sugar, butter, cinnamon and salt in a large bowl. Beat at medium speed until the mixture is crumbly. Reserve 1 1/2 cups for the topping and press the remaining mixture into the bottom of a pan to form the crust.
Whisk the sour cream, egg, remaining 3/4 cup sugar and 2 tablespoons flour in a small bowl until blended. Pour over the crust and top with the blackberries.
Sprinkle the reserved topping over the blackberries, squeezing lightly to form clumps.
Bake for 50 to 55 minutes or until light golden brown. Cool in the pan on a wire rack to room temperature.