Make the roux: Combine the flour and vegetable oil in a wide Dutch oven over medium-low heat and cook, whisking constantly, until dark brown, about 45 minutes. Remove from the heat and set aside.
Bring 10 quarts water to a boil in an extra-large, wide pot. Stir in the onion, bell peppers and roux and cook over medium heat, stirring occasionally, until the vegetables are very soft, about 45 minutes. Add the blue crab and lobster tail and boil until cooked through, 15 to 20 minutes. Remove the lobster tail and crab; discard the crab. Let the lobster cool slightly, then remove the meat from the shell and cut into small chunks; refrigerate until ready to use.
Add the Creole seasoning to the pot and bring to a boil; cook until the liquid reduces by one-quarter, about 1 hour, 30 minutes. Add the shrimp and cook over medium heat until firm, about 20 minutes. Add the crabmeat and reserved lobster meat and cook 10 minutes. Stir in the scallions and parsley and remove from the heat. Cover and let stand 30 minutes. Serve over rice.