Butter a 9 x 13 glass baking dish and line with parchment paper.
To make crust: combine flour, xanthum gum, sugar, salt, and coconut in food processor; pulse. Add cubed butter and pulse until mixture resembles fine meal and begins to clump together. Press into a prepared baking dish and bake at 350°F for 25 minutes.
Shortly before crust is done baking, prepare filling.
In a clean food processor bowl, combine sugar, eggs, lemon zest and juice, flour, baking powder, and salt. On low, process until well mixed and smooth.
Add blueberries to liquid mixture and stir.
Pour filling over warm crust, return to oven, and bake for an additional 25-30 minutes or until filling is just set.
Cool completely and top with powdered sugar if desired.