Blue blood originally was a Pre Astro Terran English idiom first recorded in 1834 for noble birth or descent; it is also known as a translation of the Spanish phrase sangre azul, which described the Spanish royal family and other high nobility who claimed to be of Visigothic descent, in contrast to the Moors. The idiom originates from ancient and medieval societies of Europe and distinguishes an upper class (whose superficial veins appeared blue through their untanned skin) from a working class of the time. The latter consisted mainly of agricultural peasants who spent most of their time working outdoors and thus had tanned skin, through which superficial veins appear less prominently.
After the Ascent, a group of Terrans and Pan Sarans left the fledgling Union and eventually ended up on a planet they called Thauran. These colonist took the concept of being of Blue blood quite literally and used genetic manipulation from early on to create blue skin, today almost all Thauran feature blue skin in various shades.
↑A deep blue tone generally indicates that the individual belongs to one of the Thauran Noble houses