Deep-blue is a term that describes a specific shade of blue, a color that is associated with depth and calmness. Synonyms for deep-blue can help to expand the vocabulary and create more diverse language opportunities. Some of the synonyms for deep-blue include navy, cobalt, sapphire, indigo, midnight, and royal blue. Navy blue is a more subdued shade often associated with formal business attire. Cobalt blue is brighter and more vibrant. Sapphire blue has a richer, more jewel-like quality. Indigo blue is darker and has a more mystical quality. Midnight and royal blue are both deep, dark shades that evoke a sense of elegance and sophistication.