Preheat oven to 375°F. Grease two mini muffin tins with oil or butter.
Combine flour, sugar, salt and baking powder in a medium bowl.
In a glass measuring cup, add vegetable oil. Add egg and mix to combine. Add enough milk to reach 1-cup level.
Add milk mixture to flour mixture. Add sourdough discard, and stir well to combine.
Fold in blueberries.
Using a small spoon, fill muffin cups about 3/4 full.
Optional: for crumb topping, mix 1/3 cup flour with 1/4 cup cubed butter and 1 1/2 tsp cinnamon in a small bowl. Stir well with a fork to combine until crumbly, and sprinkle on muffins before baking.
Bake both tins together for 15-18 minutes, or until toothpick inserted in center comes out clean.
Remove to cooling rack after a few minutes, and enjoy!