For the crumble: Preheat the oven to 375 degrees and butter a 8-by-8-inch baking dish.
Place the granulated sugar, flour, baking powder, cinnamon, salt, ginger and pistachios in a bowl and mix well. Mix in the egg and butter. Form small clumps of topping and chill for 20 minutes or until butter is firm.
For the filling: Place the sugars, flour, cinnamon, cardamom, ginger, honey, orange zest and candied ginger in a bowl and toss well. Add in the rhubarb and blackberries and toss until coated. Dump this mixture into the buttered baking dish. Dot the rhubarb with soft butter. Finish with the topping.
Place the baking dish in the oven and bake 35 to 40 minutes or until golden brown and bubbly. Remove from the oven. Cool on a cooling rack for about 20 minutes before serving. Best enjoyed warm with your favorite ice cream or whipped cream!