Will magnolia blossoms be killed off with this terrible winter?

We will have to wait and see. Magnolia hardiness has not had this severe a test. There were temps in the -20's last year so one can hope. February can be bitterly cold too, so there is a lot of winter to get through. Survival also depends on location, health in the fall before winter and other factors so as far as survival one can only give general answers. Buds should still be "ok" it is the late freezes that occur after dormancy has broken that kill the buds.