Preheat oven to 425 degrees – spray the wells of a donut pan with non-stick spray.
In a large bowl, add the sugar, vegetable oil and melted butter and mix until well combined. Add in the eggs, vanilla, milk and lemon zest and mix again.
Add in the flour and mix until combined.
Fold in the blueberries, carefully. I coated the blueberries in a spoonful of flour to prevent them from sinking to the bottom.
Using a large Ziploc bag or a piping bag, scoop the batter into the bag and cut a good 1/2 inch to an inch off the corner of your bag. You want the opening to be large enough to fit blueberries through.
Pipe the batter into the donut pan, filling 3/4 of the way full. Bake for 8-10 minutes, until the tops are a light golden brown and a toothpick comes out clean. Set to cool for a few minutes before removing from the pan, and then cool the donuts on a cooling rack completely before glazing.