In a large saucepan bring 2 quarts water to boiling. Add pasta and, if desired, 1/2 teaspoon salt. Return to boiling; cook for 5 minutes. Add frozen vegetables. Return to boiling; cook for 5 to 7 minutes more or until pasta is tender but slightly firm and vegetables are crisp-tender. Drain pasta and vegetables.
Meanwhile, for sauce, in a small saucepan melt margarine or butter. Stir in flour, 1/8 teaspoon salt, and pepper. Add milk all at once. Cook and stir until thickened and bubbly. Cook and stir for 1 minute more. Remove from heat. Stir in 1/2 cup blue cheese and the sour cream.
Pour sauce over pasta mixture. Toss to coat. If desired, sprinkle with additional blue cheese. Makes 6 side-dish servings.