Preheat oven to 350°F. Put butter in a 10-12″ cast iron skillet or similar size baking dish in oven to allow butter to melt.
Place blackberries in medium bowl, sprinkle with the 1 tbsp of sugar and gently stir to mix. Set aside.
Mix flour, sugar, baking powder, cinnamon and salt. Stir in milk until fully incorporated, do not over mix. A few lumps are ok.
Once oven is heated and butter is melted, remove skillet from oven. Pour batter mix into the center of the skillet/baking dish, letting it flow to the outer edges. Use spoon to gently even out the distribution of batter as needed.
Spoon blackberries over top of batter, leaving a ring of batter around the outside edge.
Bake about 40-50 minutes, check after 35 minutes. It should be firm, not wet looking, but spongy to the touch. If not, check every 5 minutes until desired doneness.