Preheat the oven to 375 degrees F. Place a 10-inch cast iron skillet in the oven while it's heating. Once the oven is hot, take out the skillet and carefully grease the inside with butter. Set aside.
To a medium mixing bowl, add the blackberries, granulated sugar, cornstarch, and water. Gently mix until the berries are coated.
Pour the blackberry mixture into the greased skillet. Set aside.
In a medium mixing bowl, combine the oats, brown sugar, flour, and cinnamon.
Using a fork or pastry cutter, cut the cold butter into the oat mixture until it has a small crumbly texture. Sprinkle the topping evenly over the berries.
Bake for 25 to 30 minutes or until the filling is bubbly and the topping is golden.
Remove from the oven and let rest for 5 minutes before serving. Top with vanilla ice cream if desired.