Slice about 2 inches deep through the top of the bread and continue to cut a large rectangle, being careful not to cut through the bottom of the bread. Remove top of bread and hollow out middle to create a rectangular bread bowl. Set aside.
Whisk together cream cheese, sour cream, mayonnaise, hot sauce, ranch seasoning, salt, chili powder, cumin, paprika, garlic powder and onion powder in a bowl until well combined. Stir in blue cheese, 3/4 cup Cheddar and 3/4 cup mozzarella, followed by chicken.
Spoon Buffalo chicken dip into prepared French bread and top with remaining Cheddar and mozzarella cheeses. Bake on a baking tray for 35 to 45 minutes or until cheese is melted and golden brown. Let bread sit approximately 10 minutes to set and slightly cool, then slice. Best served warm. Garnish with blue cheese if desired.