For additional meanings of "Blue (color)", please see Blue (color).
Blue was one of the four primary colors.
One symptom of the disease caused by a life prolongation project carried out on Earth Two was a blue rash. (TOS: "Miri")
Skies on Earth appeared blue when the weather was favorable. (TOS: "Who Mourns for Adonais?")
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")