Heat the oven to 350°.
Mix the margarine, blue cheese and mozzarella cheese.
Cut the bread horizontally in half. Spread cheese mixture over cut sides of bread. Reassemble the loaf.
Cut the bread crosswise into 1-inch slices. Reassemble the loaf, and wrap securely in heavy-duty aluminum foil.
Bake about 20 minutes or until cheese is melted. Serve hot.