Blue spruce are susceptible to fungal diseases such as
Cytospora canker http://extension.umd.edu/hgic/cytospora-canker-spruceand Rhizosphaeria needlecast. http://extension.umd.edu/hgic/evergreen-needle-cast-diseases-trees-and-shrubs
This past season we have received many questions on decline of blue spruce.
Blue spruce, in general, are not really well suited for our area as they are native to climates further to the north. For the above diseases all you can do is prune bare branches during dry weather and keep the trees well watered during dry periods.
No fungicides are recommended. Commercial Christmas tree plantations spray as a preventative. For homeowners, this would be a season long effort. You would have to spray when candles are elonging in the spring all the way through September.
Fertilize only according to a soil test. Results will give pH and nutrient deficiencies. This may help to reduce stress on the trees, if there is a nutrient deficiency but will not stop the disease issues. https://extension.umd.edu/hgic/topics/soil-testing
Eventually you will have to replace the trees. Take the opportunity to replace the tree with a native species matching the plant to the site and mature height and width.
Also, here is our blog post on blue spruce https://marylandgrows.umd.edu/2018/01/08/whats-wrong-with-my-blue-spruce-tre