Combine oats, brown sugar, and 1/2 cup flour in a bowl. Cut butter into oat mixture using a pastry blender until mixture is moistened and crumbly.
Spread blackberries into an 8-inch square baking dish and mix in the remaining 1/4 cup flour until berries are coated. Sprinkle crumb topping over berries.
Bake in the preheated oven until golden brown, 35 to 40 minutes.
