Make this treat on the quick by whisking together dry ingredients and wet ingredients in separate containers in advance.
1. Heat grill to 350 degrees F, set up for indirect grilling. Butter the bottom and sides of a 10-inch cast-iron skillet.
2. Whisk together flour, cornmeal, sugar, baking powder, and salt in a large bowl. Whisk together eggs, yogurt, lemon zest, vanilla, and melted butter in a separate bowl. Add wet ingredients to dry ingredients and stir to combine. Fold in 1 cup blackberries. Transfer batter to prepared pan and smooth top. Press remaining 1/2 cup blackberries into batter.
3. Grill, covered with a metal bowl and over indirect heat, rotating the pan 1/4 turn every 10 minutes, until cooked through, 35 to 40 minutes.
4. Serve warm.