Combine 4 cups blueberries, 1/2 cup white sugar, 1/2 teaspoon cinnamon, water, and lemon zest in a Dutch oven. Cook and stir over medium-high heat until mixture has thickened and has a jam-like texture, 10 to 12 minutes.
Whisk lemon juice and cornstarch together in a small bowl until smooth; stir into blueberry mixture. Add remaining 4 cups blueberries to the blueberry mixture; cook and stir until blueberry mixture is heated through, about 1 minute. Remove pot from heat and cover to keep warm.
Mix buttermilk, butter, and vanilla extract together in a measuring cup. Whisk flour, baking powder, baking soda, salt, and 6 tablespoons sugar together in a large bowl. Slowly stir buttermilk mixture into flour mixture until dough forms. Scoop dough using a small ice cream scoop or 2 large spoons and drop golf ball-sized dumplings atop blueberry mixture.
Wrap lid of Dutch oven with a clean kitchen towel to absorb condensation during cooking. Cover pot with the towel-wrapped lid, keeping towel away from heat source.
Simmer grunt until dumplings have doubled in size and a toothpick inserted in the center comes out clean, 16 to 22 minutes.
Mix 1/2 teaspoon cinnamon and 2 tablespoons white sugar together in a small bowl; sprinkle over dumplings.
