Cook macaroni according to package; drain and set aside.
Melt butter in a large oven-safe skillet over medium; whisk in flour and cook 1 minute.
Gradually whisk in milk; cook about 5 minutes until thickened, stirring constantly.
Remove from heat; stir in sour cream, 2 cups cheddar, and mozzarella until melted and smooth.
Toss chicken with buffalo sauce; fold into sauce with macaroni.
Top with remaining cheddar and blue cheese.
Broil 3-4 minutes until cheese is bubbly and golden.
Garnish with green onions; serve.