Preheat oven to 425°. In a large bowl, sift flours, baking powder, baking soda, salt, and sugar. In a separate bowl, mix blueberries with lemon zest and 1 tablespoon flour. Set aside.
Grate cold butter into the flour mixture and mix until it resembles coarse crumbs. Work fast so the butter stays cold. Reserve a dash of lemon zest.
Then, gently fold in the lemon zest and blueberry mixture. Pour in cold buttermilk, sprinkle in 1 tablespoon of thyme, and stir until just combined.
Turn the dough out onto a lightly floured countertop. Using floured hands, shape dough into a rectangle the size of a piece of paper. Fold the dough into thirds and repeat, lightly rolling and dusting the dough three times.
Pat the dough into an inch-thick rectangle slightly larger than your skillet and cut seven biscuits using a 2¾ inch biscuit cutter.
Place the biscuits in a 10.25-inch Lodge skillet, so they are just touching. Brush with buttermilk and bake for 15-20 minutes, or until golden brown and risen.
Remove the biscuits from the oven, drizzle with glaze, and garnish with reserved lemon zest and extra fresh thyme as desired.