Make the crumble: Add the sugar and lemon zest to a medium bowl. Using your fingers, rub the lemon zest into the sugar to release the oils.
Stir in the flour and add the butter, rubbing it into the flour until the mixture resembles coarse crumbs. You can also use a hand mixer and beat on low speed until a crumble forms. Keep chilled while you prepare the cake.
Make the cake: Grease your loaf pan with butter and line it with parchment paper. Set aside.
In a medium bowl, combine the flour, baking powder, and salt. Set aside. Preheat the oven to 325°F (160°C).
Add the sugar, orange zest, and lemon zest to a large bowl or the bowl of your stand mixer. Rub the orange and lemon zest into the sugar with your fingers.
Add the eggs to the citrus sugar and, using a hand mixer, beat until thick, pale, and voluminous.
Stir in the cream. Sift in the flour mixture and gently fold until combined.
Stir in the olive oil and lemon juice. Toss the blueberries in a little flour and fold them into the batter.
Pour the batter into the prepared loaf pan and scatter the crumble evenly across the top.
Bake for 60-80 minutes, or until a toothpick inserted into the center comes out clean. Dust with powdered sugar and serve.