In 2-quart saucepan, melt 2 tablespoons unsalted butter over medium heat; add 2 tablespoons all-purpose flour. Beat with whisk; cook 1 minute. Beat in 2 cups whole milk. Heat to boiling; reduce heat to simmering. Cook until sauce thickens, stirring frequently. Stir in 4 oz crumbled blue cheese until melted. Season to taste with salt and pepper.