Slowly add oil and beat for several minutes to combine.
Add yogurt, ricotta and vanilla, one at a time. Beat well after each addition.
Add one type of flour and baking powder. Mix to combine. Scrape bowl well.
Add second flour and salt. Mix to combine. Scrape bowl well.
Grease 9 x 13 pan.
Place batter in prepared pan.
Place blueberries on top of batter. Blueberries should cover the pan.
Bake for about 1 hour. Cake is done when a tooth pick inserted in the center comes out clean, or with just a couple of crumbs on it. Test at 50 minutes if you are using fresh blueberries, and 1 hour if your blueberries are frozen. Cake may jiggle slightly in center.
Let rest at room temperature for 30 minutes to firm up before cutting.