Cook pasta shells according to package instructions in a large pot of boiling salted water, then drain and set aside to cool slightly.
In a large bowl, mix shredded chicken, buffalo sauce, softened cream cheese, half of the chopped green onions, salt, and black pepper until well combined.
Spoon the chicken mixture into each cooked pasta shell, filling them generously.
Place the stuffed shells in a baking dish, sprinkle shredded cheddar cheese and crumbled blue cheese over the top.
Bake at 350°F (175°C) for 15-20 minutes until the cheese is melted and bubbly.
Garnish with the remaining chopped green onions and serve warm.