Heat oven to 350°. In a bowl, combine blackberries, rhubarb, tapioca, lemon juice, lemon zest and granulated sugar. In a separate bowl, combine oats, flour and brown sugar. Crumble in cold butter; mix with your hands until it resembles coarse crumbs.
Pour fruit mixture into a 2-quart baking dish. Top with oat mixture. Bake at 350° for 1 hour, or until topping is browned. Let stand 10 minutes before serving.
