Cook macaroni until just cooked. Cook broccoli for 3 min. in boiling salted water.
Melt margarine, add flour, mustard, salt & pepper and cook for 2 min. stirring occasionally. Gradually add milk with a whisk. Stir briskly until smooth. Remove from the heat; stir in yogurt and blue cheese. Reserve a few t. of sauce for the top and mix the remaining sauce into the macaroni, gently folding in the broccoli.
Grease an ovenproof dish, fill with the mixture, spread remaining sauce on top and bake for 20 min. at 350.