Sprinkle blackberries with white sugar and mix well. Let stand about 10 minutes. Spray an 8 x 8 baking dish with cooking spray and spread blackberries in bottom of dish.
Whisk together oats, brown sugar, ground cinnamon and flour. Cut butter into flour mixture until resembles coarse crumbs. (I cut my butter into small pieces and then cut it into the flour mixture with a fork).
Spread flour mixture over the top of the blackberries. Bake in preheated 350 degree oven for 35 minutes until brown on top.