In a skillet, brown beef and onion until beef is no longer pink and onion is tender. Drain fat. Stir in beans, salt and pepper. Place 1/3 cup of mixture and a spoonful of tomatoes on each tortilla. Roll up and place, seam side down, in a 13x9-in. baking pan.
For sauce, melt butter in a saucepan. Stir in flour until smooth. Gradually add milk and enchilada sauce; stir until smooth. Add cheese and olives; cook over medium-high until mixture boils. Pour over enchiladas. Bake at 350° for 30 minutes. Let stand a few minutes before serving.