Preheat oven to 350°F.
Stir together cream cheese, Cheddar cheese, pepper jack cheese, ranch dressing, hot sauce, pepper, and garlic powder in a large bowl until well blended.
Gently fold in chicken and half of the scallions.
Transfer to a lightly greased (with cooking spray) 13- x 9-inch baking dish. Smooth top, and sprinkle evenly with blue cheese.
Bake in preheated oven until center is hot and edges are bubbly, 20 to 25 minutes.
Remove from oven, and top with remaining scallions.
Serve with carrots, celery, and pretzel chips.
