My favorite was always "cerulean"- the color used to describe the blue sky on a clear bright day. Other sky blue words include "azure" or "cyaneous". "Cyaneous" also has a more sinister origin from the poison cyanide, which is found in the seeds and pits of apples, mangos and peaches. Cyanide was also a compound found in paint used in old paintings called "Prussian Blue" (which used to be a blue in older Crayola crayon boxes) and is used for the making of blueprints.
Other blue words are derived from gems or minerals such as "aquamarine", "lapis lazuli, "sapphire" or "turquoise".
Then there is the colder hues of blue consisting of "steel blue", "cobalt" or "chalybeous". Or even "cesious" for blue-grey.
"Royal Blue" is the color traditionally used to symbolize royalty. And "ecchymotic" is the color used to describe the color of broken blood vessels in a bruise ("black and blue").
"Mazarine" is a dark blue originating from the Mazarine Blue butterfly and "Cornflower blue" (also in the Crayola box) is rather obvious:
But I have to say, my favorite has always been "Robin's Egg Blue" (also in the Crayola box)...