When you look at your veins poking through your skin, they appear blue and you might be forgiven, if you had no anatomical knowledge, in thinking that the blood coursing through them is blue. As soon as you puncture a vein, though, the blood that spurts or trickles out is red. Does this mean that as soon as the blood comes into contact with the outside, it changes colour from blue to red because of some sort of chemical reaction?
Sorry to disappoint you but the answer is no.
It is all down to the interaction between light and subcutaneous fat. The fat which forms a barrier between your skin and what is inside – we all have it – only allows blue light to penetrate to the veins and back. Other colours, such as red, cannot make it back to your eyeballs and so the only hue you can associate with your veins is blue. Deoxygenated blood, which is what veins principally push around the body, is a darker red than oxygenated and so as a result of the way light permeates our skin will seem darker. If you look at different veins around your body you will see that they are not uniform in colour – this is because the diameter and thickness of the walls allow more or less of the blue light to reach your line of vision.
The reason for this rather discursive explanation in what is meant to be an etymological discussion is to nail on the head the idea that members of the royal family and the aristocracy have blue blood. Now that any Mike, Kate or Meghan – a distinctly unroyal trio, if you ever saw one – can marry into the royal family, it would be hard to defend seriously the proposition that our so-called betters have blood of a different colour coursing through their veins. When royals and aristocrats studiously intermarried within their own charmed circle, it might have been possible to hoodwink the masses into thinking so but the odd execution of a royal – to be encouraged in my view – would have scotched that theory.
So why do we call royals blue-blooded?
Blame the Spanish and subcutaneous fat. The proud boast of some of the oldest and proudest families in Castile was that their stock was pure, having resisted the temptation to intermarry with Moors, Jews and the like. The consequence was that their skin colouration was lighter than the other indigenous population and their veins seemed darker. This phenomenon gave rise to the term sangre azul or blue blood.
By the 19th century the term established itself in the English language. Save for making a direct translation of it, we did little else. Maria Edgeworth testified to its origin in her 1834 novel, Helen; “from Spain, of high rank and birth, of the sangre azul, the blue blood.” By the time Anthony Trollope came to write The Duke’s Children in 1880, it was a familiar sobriquet for the oldest and most aristocratic families to be used without the aid of a gloss; “It is a point of conscience among the – perhaps not ten thousand, but say one thousand of bluest blood – that everyone should know who everybody is…It is a knowledge which the possession of the blue blood itself produces. “
It takes one to know one, it seems.