Heat oven to 325 degrees. Line the bottom of a 9-inch springform pan with parchment paper. Butter the paper and sides of the pan. Wrap two layers of foil under the pan, and place it on a baking sheet.

In a medium bowl, mix rhubarb and blueberries, cornstarch and 1/2 cup granulated sugar.

Mix the brown sugar and 1/4 cup butter in a pan over medium heat. Whisk until smooth and bubbling for 2 minutes. Pour the brown-sugar mixture into the cake pan and set aside.

Sift together the flour, baking powder, baking soda and salt.

Whip 1 cup butter in a mixer for 2 minutes. Add remaining 1 cup sugar with orange zest, and cream together until it is light and fluffy, for 4 minutes. Add the vanilla and almond extract and mix well.

Add the eggs, one at a time. Mix in the sour cream and orange juice (it might look curdled). Slowly add in the flour, and mix on low for 2 minutes.

On top of the brown sugar mixture in the springform pan, spoon in the rhubarb & blueberry mixture and its juices. Next, spoon in the batter so it covers all of the rhubarb.

Bake for 1 hour and 15 minutes, or until the top of the cake is firm to touch, and a toothpick stuck in the middle comes out clean.

Place the pan on a wire rack, and cool for 20 minutes. Run a knife around the cake, place a plate on top of the pan and flip it upside-down. Release the cake from the pan while still warm or else it will stick. Allow to cool for another 15-20 minutes. Right before serving, dust with powdered sugar.