Preheat your oven to 350 F (180 C) degrees. 

Grease a springform pan (8″ or 9″) with a bit of coconut oil and line with parchment paper. Grease the parchment paper as well.

Zest one of the blood oranges and reserve the zest. Peel the blood oranges and slice into ½-inch rounds. Evenly layer the blood orange slices on the bottom of the springform pan and sprinkle some black sesame seeds on top. 

In a small saucepan, combine ⅓ cup of sugar with 3 tablespoons of water. Bring the mixture to a simmer over medium heat, and simmer until the sugar is dissolved, about 3 minutes. Pour the sugar mixture evenly over the orange slices in the cake pan.

In a large mixing bowl, whisk together the polenta, flour, ½ a cup of sugar, baking powder, baking soda, and salt. 

Make a well in the center of the dry ingredients, and add the coconut milk and melted coconut oil. Using a spatula or a wooden spoon, stir together the wet and dry ingredients until just combined; be careful not to over mix.

Add the black sesame seeds and reserved blood orange zest to the batter and mix until just incorporated.

Pour the batter over the sliced blood oranges. Smooth out the batter with the spatula or back of a spoon. 

Bake until a toothpick inserted into the center of the cake comes out clean, around 45 minutes.

Allow the cake to cool in the pan for 20 minutes, then unmold and flip the cake onto a plate/platter so that the bottom with the oranges is now facing up. 

Allow to fully cool, then slice and serve.