Make this treat on the quick by whisking together dry ingredients and wet ingredients in separate containers in advance.
Heat grill to 350°F, set up for indirect grilling. Butter the bottom and sides of a 10-inch cast-iron skillet.
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.
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.