Preheat oven to 375 F and line a baking sheet with parchment paper.
Combine coconut and cassava flour in a large bowl. Use a fork or pastry cutter to cut the coconut oil into the flour until the mixture is crumbly, then cut in the sugar. Stir in baking soda, cream of tartar, egg, vanilla, and coconut milk until well combined, then stir in blueberries.
Wait 5 minutes for the coconut flour to absorb, and then scoop the batter onto the center of the baking sheet and form a large circle.