Make flax egg by adding 2 tablespoons of flax egg mixture with 6 tbsp of water. Set aside for 5 minutes until the consistency of an egg.
In a small bowl whisk together flour, sugar, baking soda, baking powder, and salt. (set aside)
In a medium bowl whisk together milk, flax egg, vanilla, lemon juice, and butter.
Toss frozen blueberries in 1 tbsp of flour.
Slowly whisk dry ingredients into the wet mixture until smooth. Fold in blueberries.
Using a medium ice cream scoop, scoop muffin mixture into baking cups, filling them to the top.
Sprinkle crumb topping on to muffins.
Bake for 25-30 minutes. Check muffins at the 25-minute mark by inserting a toothpick into the center of the muffins. once the toothpick comes out clean, remove muffins from the oven and allow them to cool for 10 minutes.