Preheat the oven to 425℉. Pour the oil into an 8 inch cast iron skillet. Bake for about 10 minutes to heat the oil.
In the meantime prepare the batter. In a large bowl add the cornmeal, flour, baking powder, baking soda and salt. Stir to combine with a fork.
Add the eggs, buttermilk and honey. Stir to combine. Add the melted butter and stir.
Gently fold in the blackberries. If you stir too hard then you will break the berries and the juice will leak into the batter.
Once the skillet is heated remove from the oven and carefully pour the batter into the oil, it will sizzle. Add extra berries to the top of the cornbread if desired.
Reduce the heat down to 375℉ and bake the cornbread for about 40 minutes, the edges will be slightly browned and a toothpick inserted into the center will come out clean.
Allow to cool in the skillet about 10 minutes, cut and serve. Top with butter and honey if desired.