Wash the blood oranges thoroughly and place them in a saucepan. Cover with water and boil for 2 hours, or until completely tender.
Drain the oranges and let them cool enough to handle.
Trim off the stem ends, cut the oranges into quarters, remove any seeds, and puree the entire fruit (including the peel) in a food processor until smooth.
Preheat oven to 375°F. Grease a 9-inch springform pan and line the bottom with parchment paper.
In the bowl of a stand mixer, beat the eggs and sugar on medium-high speed for 4–5 minutes, until pale and slightly fluffy.
Mix in the blood orange puree until combined.
In a separate bowl, whisk together the almond flour, baking powder, and salt.
Gently fold the dry ingredients into the wet mixture until fully incorporated.
Pour the batter into the prepared pan and smooth the top.
Sprinkle with sliced almonds, if using.
Bake for 25-30 minutes, or until the center is set and a toothpick inserted in the middle comes out mostly clean.
Cool in the pan for 20–30 minutes.
Run a knife around the edges, release the springform, and transfer to a serving plate.
Dust with powdered sugar before serving.