Heat broiler. Bring a large pot of lightly salted water to boiling. Add pasta and cook 8 minutes, or as per package directions; drain.
Meanwhile, season chicken with 1/2 teaspoon of the celery seed, 1 teaspoon of the paprika and the salt. In a large skillet, heat oil over medium-high heat. Brown chicken, 3 to 5 minutes, until cooked through; set aside. Lower heat to medium. Add celery to same skillet; sauté 4 minutes. Add garlic; cook 1 to 2 minutes. Set aside with chicken.
In saucepan, melt 3 tablespoons of
the butter over medium heat. Sprinkle
in flour and mustard powder, whisking
for 2 minutes. Pour in milk and sauce,
whisking until it boils. Simmer
3 minutes. Remove from heat; stir in
cheddar, 1/2 cup of the blue cheese and
remaining teaspoon paprika.
Add pasta, celery, garlic and chicken. Place in a broiler-safe dish.
Microwave remaining tablespoon butter for 30 seconds. Stir in bread crumbs and remaining 1/2 teaspoon celery seed; sprinkle over dish with remaining 1/4 cup blue cheese. Broil 2 to 3 minutes, or until browned.