Cook macaroni according to package directions, drain and set aside.
In a large oven-safe skillet, melt butter over medium heat and whisk in flour, cooking for 1 minute.
Gradually whisk in milk and cook until thickened, about 5 minutes, stirring constantly.
Remove from heat and stir in sour cream, 2 cups cheddar cheese, and mozzarella until melted and smooth.
Toss shredded chicken with buffalo sauce, then fold into the cheese sauce along with cooked macaroni.
Top with remaining cheddar cheese and blue cheese crumbles.
Broil for 3-4 minutes until cheese is bubbly and golden.
Garnish with sliced green onions before serving.