1Heat oven to 350 degrees.
2Grease and line base and sides of a 9-inch-square cake pan with baking parchment, and grease the parchment.
3Sprinkle 1/3 cup sugar over base of pan, and cover evenly with blueberries.
4In a large mixing bowl, combine eggs, 1 cup sugar and orange zest. Whisk until pale and thick.
5Add orange juice and oil, and whisk until blended.
6In a separate bowl, whisk together polenta, flour, baking powder and salt.
7Add flour mixture to egg mixture, whisking until smooth.
8Pour into prepared pan.
9Bake for about 45 to 55 minutes, until golden brown and springy to the touch, and a toothpick inserted near the center comes out clean.
10Cool cake on a rack for about 5 minutes.
11Carefully invert cake onto a serving plate, and slowly peel off parchment paper.
12Serve warm or at room temperature.
This is a variation on a recipe in the New York Times, https://cooking.nytimes.com/recipes/271-blueberry-polenta-upside-down-cake, as cooked by Heidi Welsh, Boonsboro, MD, using her own fresh-picked garden blackberries. You can make this with blueberries or raspberries. Polenta flour is essentially the same as cornmeal.