Preheat the oven to 350 (F). Grease a 9-inch round cake pan with butter and spray with cooking spray.
In a small saucepan over medium heat, melt 3 tablespoons unsalted butter. Add 2/3 cup brown sugar and 2 teaspoons lemon juice, stirring until the sugar melts (about 2-3 minutes). Scrape the mixture into the bottom of the prepared cake pan.
Grate 1/2 teaspoon zest from one of the oranges, then slice off the tops and bottoms of 2 blood oranges. Peel the oranges so that the rind and pith no longer remain. Slice each orange crosswise into 1/4-inch thick wheels, discarding any seeds. Arrange orange wheels on top of the brown sugar mixture in a single, tight layer.
In a large bowl, whisk together 1/2 teaspoon orange zest, 1 cup cornmeal, 1/2 cup flour, 1 1/2 teaspoons baking powder, and 1/2 teaspoon salt. Set aside.
In the bowl of a freestanding electric mixer with a paddle attachment, cream together 2 sticks unsalted butter with 1 cup granulated sugar until light and fluffy, about 3 minutes. On a low speed, beat in 4 eggs, one at a time, only adding an egg after the previous egg has been fully incorporated. Once all the eggs have been incorporated, beat in 1/3 cup sour cream and 2 teaspoons vanilla until just incorporated
Sprinkle in the mixture of dry ingredients (from the fourth step) over the mixture of wet ingredients (from the fifth step) and use a rubber spatula to fold the dry mixture into the wet ingredients by hand. DO NOT OVERMIX.
Scrape the batter into the cakepan over the orange slices in the cake pan. Transfer to the oven and bake until the cake is golden brown and a tooth pick inserted in the center emerges clean, about 40 to 50 minutes. Cool the cake in the pan on a cooling rack for 10 minutes, then run a knife along pan's edges to loosen it and invert onto a platter. Cool completely before serving.