Preheat oven to 350°F. Put butter in a 6" cast iron skillet or similar size baking dish in oven to allow butter to melt.
Place blackberries in small bowl, sprinkle with the 1 tsp 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 minutes, check after 30 minutes. It should be firm, not wet looking, but spongy to the touch. If not, check every 5 minutes until desired doneness.