To make your own almond flour, pulse 1 cup of sliced or slivered almonds at a time in a food processor until finely ground.
Preheat oven to 375 degrees. Line a standard 12-cup muffin tin with baking cups, or lightly brush with butter. In a small bowl, whisk together almond flour, cornmeal, baking powder, and salt. Beat butter with sugar and lemon zest on medium-high speed until pale and fluffy, about 5 minutes. Add eggs, one at a time, beating well after each addition. Add dry ingredients and beat until just combined. Fold in 1 cup blackberries.
Divide batter evenly among muffin cups. Scatter remaining 1/2 cup blackberries on top; sprinkle with sugar. Bake until golden and tops spring back when lightly touched, about 25 minutes. Let cool 5 minutes in tin, then run an offset spatula around edges and transfer to a wire rack; let cool completely.