The Novan Jamin with his blue colored face
Blue was one of the three primary colors.
The Novans used blue and yellow color to paint their faces. (ENT: "Terra Nova")
The Andorians were known to have blue skin. The Aenar called them "blue skins". (ENT: "The Andorian Incident", "The Aenar")
One symptom of the disease caused by a life prolongation project carried out on Miri was a blue rash. (TOS: "Miri")
Skies on Earth appeared blue when the weather was favorable. (TOS: "Who Mourns for Adonais?")
In the Earth fairy tale The Kingdom of Elysian, the color blue was associated with King Ridley and was used for identifying his territory on maps of the Kingdom of Elysian. (SNW: "The Elysian Kingdom")
On Federation star charts of the 23rd century, 24th century, and 32nd century, the color blue was used for Federation territory and locations. (SNW: "Strange New Worlds"; PIC: "Maps and Legends"; DIS: "Scavengers")
On the same charts, from the 23rd century, blue was used, as well, for Tholian territory and locations. (SNW: "Children of the Comet")
In the 2370s, isolinear rods were color-coded with labels that correspond with what particular information they pertain to. One of the colors used was blue. (DS9: "Shadowplay")
In the 32nd century, the holographic bird alarm clock owned by Aditya Sahil would change its color daily, with blue being one of the colors. On Hima, there was a species of blue flying insects, a species of fish with blue stripes and matching appendages, and a species of blue water plants. (DIS: "That Hope Is You, Part 1")