Where did the expression 'blue blood' come from?

The inbred European aristocracy has always admired paleness - Queen Elizabeth I used to cover her face with white lead powder - and the veins of very pale-skinned people show blue under the skin. Hence the notion that the idle high-born have blue blood. And they do, too! Unless it is carrying oxygen or exposed to the air, all blood is blue.

Editing this answer:

All blood is definitely NOT blue! All blood is red. There are approximately 35 trillion red blood cells circulating in the human body at any given time, so they lend some of their red colour to blood only by virtue of their sheer volume. Red blood cells only have a lifespan of approximately 4 months before they 'die' by being broken up in the spleen and replaced by new red blood cells. As new red blood cells mature, they increase their production of hemoglobin (iron mixed with protein), which is the true red pigment of blood. All blood, whether carrying oxygen (going through the lungs & beyond to the rest of the body) or carbon dioxide (heading back to the lungs) is red.

The rest of the original answer is correct. Royalty has always sought to differentiate itself from the common man, and one way of doing that in olden times was to keep royal skin as pale and as untouched by the sun as possible - since the common man, usually a farmer, would be quite browned by working outdoors.

The term "blue blood" originated in Spain. It was used to differentiate between people with pale skin (Europeans) from others. The veins appeared more blue due to the lighter skin. The term was carried over from Spanish to English in the early 1800's.

Ameer

spanish

The first correction is also right, but left out of this PC version is who the "others" were. The lighter skin population were Spaniards who used their fair skin to prove that the bloodline of their family was not tainted by the Moors during their occupation. This was used as a way to segregate and possibly even prosecute individuals who were not fair skin. These were the "others". This way of thinking in Spain carried on even into the occupation of lands within the new world.

Blue blood is black blood (1100-1848) by Egmond Codfried. Blue men were the black and brown complexioned Europeans, the first Europeans before the whites arrived 6000 years ago. From 1100-1200 their descendents called themselves blue bloods and considered themselves the true Europeans. blue blood is the identity of the oldest and highest nobility. They enforced intermarriage because the brown and black complexion meant superiority over whites. Hence also the images of Black Madonna's, which were created around the same time, which meant that god was Black. This also goes for the Black King Balthazar as the Black nobility brought Christianity to Europeans.