You are correct that Colorado blue spruce, Picea pungens, is "highly suseptible" to Rhizosphaera needle cast of spruce, a fungal disease. (see link) 

https://u.osu.edu/ornamentaldiseasefacts/nursery/rhizosphaera-needle-cast/dig-deeper/

"Needle cast symptoms can also be caused by pests, such as the pine needle scale or the spruce spider mite, or an abiotic stress to the tree, such as drought. Each of these diseases/disorders has specific and unique symptoms, so make sure to closely inspect the tree before making management decisions.

If uncertain, samples can be sent to a diagnostic laboratory for confirmation. Information on The Ohio State University laboratory can be found on the C. Wayne Ellett Plant and Pest Diagnostic Clinic website" Here's the directions for Landscape, Woody Ornamental Plants https://ppdc.osu.edu/submit-sample/landscape/woody-ornamental-plants  Contact the lab on the best day to send and if they can diagnose from a digital/photograph instead. 

https://u.osu.edu/ornamentaldiseasefacts/nursery/rhizosphaera-needle-cast/what-is-it-what-to-do-about-it/

Whenever using chemical controls, always follow the label directions.