Preheat oven to 400 degrees F (200 degrees C) and grease muffin tin or use paper liners.Combine 1½ cups flour, ¾ cup sugar, salt and baking powder in a bowl and whisk to mix.In a 1-cup measuring cup, add vegetable oil, the egg and enough milk to fill the cup. Lightly mix to break the egg.Add the wet ingredients to the dry ingredients and mix til nearly all the flour is combined. Add blueberries and fold them into the batter.Fill muffin cups right to the top, and sprinkle with crumb topping mixture.In a medium sized bowl, mix together ½ cup sugar, â…“ cup flour, ¼ cup butter, and the lemon zest.Mix with fork, and sprinkle generously over muffins before baking.Bake for 20 to 25 minutes in the preheated oven, or until done.