Prep the peppers: Preheat your oven to 400°F (200°C). Lightly oil a baking dish. Slice the tops off the bell peppers and remove seeds and membranes.
Rub the outsides with olive oil and season the insides with a pinch of salt and pepper.
Par-bake for tenderness: Place the peppers upright in the dish and bake for 10 minutes. This softens them slightly so they bake evenly once filled.
Make the filling: In a large bowl, combine shredded chicken, Greek yogurt, buffalo sauce, celery, red onion, garlic powder, onion powder, smoked paprika, and a pinch of salt and pepper. Stir until everything is well coated.
Taste and adjust heat by adding more buffalo sauce if needed.
Add the cheese: Fold in half of the shredded cheese and the blue cheese (if using). Reserve the remaining cheese for topping.
Stuff the peppers: Carefully spoon the buffalo chicken mixture into each par-baked pepper, packing it down slightly. Top each with the remaining shredded cheese.
Bake until bubbly: Return to the oven and bake for 18–22 minutes, or until the peppers are tender and the filling is hot and bubbly.
Finish and serve: Let the peppers rest for 5 minutes.
Garnish with chopped parsley or chives. Drizzle with a little ranch or extra buffalo sauce if you like more kick. Serve warm.