The term "blue sky" is often used to describe a clear and beautiful sky on a sunny day. However, there are many other synonyms for this expression depending on the specific context or mood. Some of the alternative phrases include "azure canopy", "cerulean heavens", "sapphire expanse", "cobalt dome", "turquoise vault", "navy firmament", "indigo blanket", and "sky-blue vista". Additionally, there are some idioms and metaphors that use the color blue to describe various emotions and situations, such as feeling "blue" or having "the blues", referring to sadness or melancholy.