Preheat oven to 350 degrees, and grease or line a 9 by 9 inch baking pan.
In a medium bowl, mix flour, salt, baking soda, and baking powder. Set aside.
In another bowl, whisk together oil and sugar. Then add eggs and whisk until creamy. Add buttermilk, milk, and vanilla and whisk to combine.
Add the flour mixture into the batter and stir until just combined. Do not over mix.
Pour the batter evenly into the baking dish, and pour the blueberries over the top. Press them down gently so they are set down into the batter. They will sink some as they bake.
Bake at 350 degrees for 20-23 minutes. The edges will be golden brown, and a toothpick inserted into the center will come out clean when the cake is done.