1Heat oven to 425°F. Cook and drain macaroni as directed on package, using minimum cook time.
2Meanwhile, in 3-quart saucepan, melt butter over low heat. Stir in flour and mustard with whisk; cook and stir until smooth. Gradually add milk. Heat to boiling, stirring constantly. Boil 1 minute until mixture is hot and bubbly. Add Cheddar cheese; stir until cheese is melted and sauce is smooth. Season with salt.
3Gently stir cooked macaroni into cheese sauce. Stir in chicken. Spoon into ungreased 8-inch square (2-quart) glass baking dish. Sprinkle celery and blue cheese over macaroni mixture. Drizzle with Buffalo wing sauce.
4Bake uncovered 5 to 10 minutes or until bubbly and blue cheese is melted. Serve immediately.