Preheat oven to 375 degrees and coat a 10-inch cast iron skillet with baking spray or melted butter.
In a large bowl, combine blackberries, sugar, cornstarch, lemon juice, lemon zest, spiced rum, and browning. Stir until blackberries are coated. Set aside while you prepare the biscuit topping.
To prepare the biscuit topping, whisk together flour, sugar, and baking powder.
Add melted butter and whole milk to the flour mixture. Stir until a batter forms (do not over-mix).
Pour blackberries into the skillet and drop dollops of batter on top. Space them out evenly for visibility to blackberries.
Place in the oven on the middle rack for 40-45 minutes until blackberries are bubbling and topping is golden brown.