Cate Blanchett is used to costumes bordering on disguise on the big screen—layers of thick white makeup for her star turn in the “Elizabeth” movies, or a wig and leather jacket for her portrayal of a young Bob Dylan in the upcoming “I’m Not There.” But it’s still surprising to picture her serving breakfast at home wearing an actual mask. Donning a white cotton, full-face moisturizing cover akin to a soft hockey mask is part of her skin care ritual, says M...