This is most likely due to cytospora canker, a very common disease of mature spruce.  If you look at some of the dying lower branches, you will probably see the tell-tale signs of the infection - bluish white blotches of resin on the bark. Unfortunately we do not have any effective sprays to prevent the disease or cure it.  The only management advice is to keep the tree healthy to reduce the spread of an infection.  The disease is very common on blue spruce trees over 20 years old.