Preheat your oven to 375 degrees F.
In a large bowl, combine the blueberries, flour, granulated sugar, brown sugar, cinnamon, nutmeg, and salt. Toss to coat the blueberries evenly.
Pour the blueberry mixture into a 10-inch cast iron skillet.
In a separate bowl, combine the flour, oats, brown sugar, and cinnamon. Mix well.
Add the cubed butter to the dry mixture and use a pastry cutter or your fingers to work the butter into the mixture until it resembles coarse crumbs.
Sprinkle the topping evenly over the blueberries.
Bake the skillet blueberry crisp for 35-40 minutes, or until the topping is golden brown and the blueberries are bubbling.
Remove the skillet from the oven and let it cool for 5-10 minutes before serving.
Serve the Cast Iron Skillet Blueberry Crisp warm with vanilla ice cream or whipped cream, if desired.