Preheat oven to 375 degrees F (190 degrees C) and place rack in the center of the oven. Butter, or spray with a non stick cooking spray, a 9 inch (23 cm) deep dish pie plate or an 8 x 8 x 2 inch (20 x 20 x 5 cm) baking dish. 

For Topping:  Place all the topping ingredients (flour, sugar, cinnamon, salt, butter, and oats) in a food processor and process until the mixture is crumbly (looks like coarse meal) and there are no large pieces of butter visible. (This can also be done in a large bowl with a pastry blender, two knives or your fingertips.) 

For Filling: Place the rhubarb and blackberries in a large bowl and toss with the brown sugar. 

In a separate bowl, whisk together the orange (or apple) juice, cornstarch (corn flour) and vanilla. Add this mixture to the rhubarb mixture and toss to coat. Transfer the fruit to your baking dish and sprinkle the streusel topping evenly over the fruit. Bake for approximately 30-35 or until the topping is crisp, golden brown and bubbly. Remove from oven and place on a wire rack to cool for about 30 minutes before serving.

Serve with softly whipped cream or vanilla ice cream.  Refrigerate leftovers and reheat before serving.

Makes 4 - 5 servings.