Preheat the oven to 400°F and grease a 6 cup muffin tin with vegan butter, coconut oil, or cooking spray (or line with cupcake papers for oil-free).
Add the streusel ingredients to a small bowl and mash together using a fork or your hands until it holds together and looks crumbly. Set aside.
In a small bowl, combine the ground flax seeds and warm water. Give it a quick stir and set it aside to thicken for at least 5 minutes.
If your blueberries are large, give them a quick chop. If you're using frozen berries, toss them in a tablespoon of brown rice flour.
In a large mixing bowl, add the oat flour, brown rice flour, tapioca starch, almond flour, baking powder, and salt. Stir to combine.
In a medium bowl, combine the flax mixture, almond milk, maple syrup, melted vegan butter/oil, and vanilla extract.
Add the wet ingredients to the dry ingredients and stir until just combined. Finally, carefully fold the blueberries into your batter. If you're using frozen berries, be careful not to overmix it or the batter will turn purple.
Divide the batter equally between 6 muffin cups, scooping by ¼ cup and filling each muffin cup to just over half-full. Sprinkle the tops generously with streusel mixture.
Bake the muffins for 22 to 25 minutes, or until tops are lightly golden. Cool in the muffins in the pan for 10 minutes, then remove them from the pan and place them directly on the rack to cool completely.
Store the muffins in an airtight container at room temperature for up to 3 days, or freeze for later.