Buffalo Chicken Mac and Cheese



2 1/2 cups uncooked elbow macaroni
1/4 cup unsalted butter
1/4 cup Gold Medal™ all-purpose flour
1/2 teaspoon ground mustard
2 cups whole milk
2 cups shredded Cheddar cheese (8 oz)
Salt to taste
1 cup chopped cooked chicken
1 stalk celery, cut in half lengthwise, then cut into 1-inch pieces
1/4 cup crumbled blue cheese (1 oz)
1/4 cup Buffalo wing sauce