Preheat oven to 350 degrees. Butter a 2-Qt. baking dish.
In a medium bowl whisk together 1 cup of sugar, cinnamon, flour, buttermilk, and vanilla. Then add in melted butter.
Pour the buttermilk mixture into the baking dish. Sprinkle the blackberries in a single layer over the batter. Dust the berries with 1/4 cup of sugar. Bake 40 minutes then sprinkle with cinnamon-sugar. Continue baking an additional 10 - 15 minutes until bubbly and golden brown.
Allow to rest or not. Serve with vanilla ice cream or whipped cream.