I think it’s probably originated because the people it referred to had such thin white skin the blue-looking veins of blood showed through.

It comes from the early 1800’s as a Spanish phrase “sangre azul” (translated … blood blue), which described the Spanish royal family and other high nobility claimed to be of Visigothic descent. The Moors (Islamic) had taken over the Iberian Peninsula (Spain & Portugal). If I remember, it was like 800 years. Although Moorish was not an ethnic race, they tended to be darker skinned compared to the Visigoth. The Christian Visigoth Hispani were lighter skinned, and you could see their veins through the skin easily. They regained control and it was said that their lineage was the true ruling class. TRUE BLUE and BLUE BLOOD come from this time in history when the Catholics regained control of this area. They also seemed to be trying to distinguish themselves from non-Arian Catholics. Arian was a philosophy that the Holy Trinity (GOD, THE SON, AND THE HOLY GHOST) where separate or at least “begotten” separately. Prominent Emperors such as Constantius II, and Valens were Arians, as well as prominent Gothic, Vandal and Lombard warlords both before and after the fall of the Western Roman Empire. Thus, their claim as being the ruling class more authoritative. Curiously, the Arians were considered more tolerant of other religions and races. Some will say that it distinguished classes because the peasants worked outside so they were more tan, but that just doesn’t hold up if you consider winter time – when tanning would not be as big of a factor. I’ll put some links below if you are just wondering. The Arian bit is kinda interesting, because it is nothing like the notions of an Aryan nation like Hitler or the skin heads portray.

To quote from the article:

It’s a direct translation of the Spanish sangre azul. Many of the oldest and proudest families of Castile used to boast that they were pure bred, having no link with the Moors who had for so long controlled the country, or indeed any other group. As a mark of this, they pointed to their veins, which seemed bluer in colour than those of such foreigners. This was simply because their blue-tinted veins showed up more prominently in their lighter skin, but they took it to be a mark of their pure breeding.

So the phrase blue blood came to refer to the blood which flowed in the veins of the oldest and most aristocratic families. The phrase was taken over into English in the 1830s. By the time Anthony Trollope used it in The Duke’s Children in 1880, it had become common:

It is a point of conscience among the — perhaps not ten thousand, but say one thousand of bluest blood, — that everybody should know who everybody is. Our Duke, though he had not given his mind much to the pursuit, had nevertheless learned his lesson. It is a knowledge which the possession of the blue blood itself produces. There are countries with bluer blood than our own in which to be without such knowledge is a crime.

Blue was in continental europe a colour connected with nobility/royalty since the period of Celts (and indirect testimony of that and more presicely in Britain can be found in Plato’s Timeus in the end of Atlantis’ tale). From that it was only a`short step to supose that nobility has also blue blood to diferenciate itself from the rest of the mankind