Preheat oven to 350F. Spray a 13x9-inch baking dish with cooking spray; Set aside.
Mix together the cheddar and pepper jack cheese. Set aside.
Cook the macaroni in salted water per the package directions until al dente. Drain well and set aside.
Melt butter in a 6 quart Dutch oven or heavy bottomed pot over medium heat. Add celery and green onions. (reserve some green onions for garnishing.) Cook for 2 minutes or just until beginning to soften. (Using a large pot will double as the mixing bowl.)
To the butter, whisk in flour, garlic salt and black pepper. Cook stirring constantly for 1 minute.
Gradually whisk in milk. Increase the heat to medium-high and bring to a gentle boil, stirring constantly. Once the sauce bubbles, immediately lower the heat to low. Let simmer uncovered gently bubbling for 3-5 minutes, stirring occasionally to prevent sticking.
To the pot add the hot sauce, cream cheese, dry ranch seasoning and 2 cups of the shredded cheese mixture. Stir until cheese is melted and mixture is smooth. Remove from the heat.
To the Dutch oven add cooked macaroni, chopped chicken and bleu cheese crumbles. Stir gently to coat well.
Pour half of the macaroni mixture into the baking dish. Sprinkle with 1 cup of the remaining shredded cheese. Repeat pouring the macaroni mixture and sprinkle the top with the final cup of shredded cheese. Dot the top with butter. (about 2 Tbsp)
Bake, uncovered, for 35-45 minutes or until the top is golden and it's bubbly around the edges.
Let stand 5 minutes then serve drizzled with additional hot sauce, ranch dressing, bleu cheese crumbles and garnished with green onions.