Preheat the oven to 375 F, and butter a 9x13 baking dish.
Combine the brown sugar, baking powder, salt, and flour in a bowl. Mix until incorporated, then cut in the butter and egg with a pastry cutter or forks. Mixture should be crumbly.
Pat ⅔ of the mixture into the prepared pan. With the remaining ⅓, add the oats and mix to combine.
In a separate bowl, combine the rhubarb, blackberries, white sugar, and cornstarch. Mix well, then assemble evenly over the prepared crust.
Pour the remaining crust/oat mix over the fruit.
Bake for 45 minutes, or until the top is turning golden. Remove and let cool completely before slicing and serving.
Recipe by The Kitchen Paper at http://c02.ddd.mwp.accessdomain.com/blackberry-rhubarb-oat-crumble-bars/