Preheat oven to 375 degrees. Stir blackberries, 1/4 cup sugar, 2 1/2 tablespoons flour, and the lemon juice in a bowl; set aside.
Put butter, cinnamon, salt, and remaining tablespoon sugar into a small bowl. Stir vigorously with a rubber spatula until creamy. Stir in cookies, oats, and remaining 1 1/2 teaspoons flour. Work mixture through fingers until it forms coarse crumbs ranging in size from peas to gum balls.
Butter four 5-ounce ramekins (about 3 1/2 inches in diameter). Divide blackberry mixture among ramekins; transfer to a rimmed baking sheet. Sprinkle with crumb topping. Bake until juices are bubbling and topping is golden brown, 20 to 25 minutes. Let cool on a rack 20 minutes before serving.