In a medium mixing bowl, combine blue cheese dressing, sour cream, and cream cheese.
In a separate bowl, combine diced chicken, buffalo sauce, and garlic powder. Add half of the chicken mixture to cream cheese mixture along with celery, green onions, and half of the cheddar and mozzarella cheeses.
Spread mixture into an 8x8 baking dish. Top with remaining buffalo chicken and remaining cheese. Bake for 25 minutes or until heated through and cheese is melted.
Notes
Ranch dressing can be substituted for blue cheese dressing, if desired.