Zest one of the oranges. Slice half of the other orange and set aside. Squeeze remaining half. It should yield enough juice for ¼ cup.
Position a rack in the lower third of your oven and preheat to 325ºF. Spray the bottom and sides of a 9-inch springform pan or cake pan with the cooking spray. Line the bottom of the pan with a round of parchment paper and spray the parchment as well. If using a springform pan, place on a rimmed baking sheet to catch any drips.
Melt dairy-free butter and add ½ cup sugar. Mix well. Pour into the bottom of the cake pan and place orange slices on top.
In a large bowl, whisk together the olive oil, ½ cup sugar, blood orange zest and juice, and egg yolks to combine. Sift in the almond and tapioca flours, baking powder, and salt. Stir to combine.
Place the egg whites and cream of tartar in the clean bowl of a stand mixer fitted with the whip attachment. Beat on medium-high speed until foamy, 30-60 seconds. With the mixer running, gradually sprinkle in the remaining ¼ cup sugar, 20-30 seconds. Continue to whip until the egg whites are shiny and hold firm peaks when you lift the whip out and hold it upside-down, 2-4 minutes.
Fold one-third of the whipped egg whites into the batter until just combined. Fold in the remaining egg whites until just combined and no streaks remain. Scrape the batter into the prepared pan and smooth the top.
Bake the cake for 40–45 minutes or until deep golden on top, beginning to pull away from the sides of the pan, and the top springs back when pressed lightly. (If top of the cake darkens too quickly, tent with a piece of aluminum foil.) Loosen the edges with a small, offset spatula and release the sides if using a springform pan. Invert the cake onto the wire rack and peel away the parchment, being careful to have the orange slices stay in place. Let cool completely. Dust with powdered sugar. Serve with cream and berries if desired.