We all think we know what the phrase 'blue-blooded' means - it's generally taken to refer to the blood of people of aristocratic heritage, and is not a literal term. When we say that someone is 'blue-blooded', we just mean that they are from an old and aristocratic family. But where does this evocative term come from?
The site phrase finder has a very simple explanation. They attribute the phrase 'blue blood' to a literal translation from the Spanish 'sangre azul' - used to refer to some of the oldest families in Castile who claimed never to have intermarried with the Moors, Jews or any other waves of immigration that Spain experienced over the last thousand years or so.
There is further speculation that the 'sangre azul' phrase originated due to the blue veins of these proud old aristocratic families, which would be much more noticeable with their pale complexions than on the arms and legs of descendants from the various ethnic groups which have intermarried their way across Spain over the centuries.
This ties in, of course, with the peculiar reversal of attitudes that we now associate bronzed, tanned skin with wealth and propserity, whereas not so very long ago women in particular would poison themselves with arsenic to try and achieve a properly pale and pallid complexion!
The phrase has caught on and is often used with reference to the English aristocracy. It carries associations of old age, fustiness and conservatism. Fox hunting and flogging the house boy. In 2003, The Hindu pubished an article claiming that the phrase is used to describe certain institutions in the United Kingdom, with specific reference to Oxford and Cambridge University, which are apparently described as 'blue brick'. It's fair to say that this is not really the case on any widespread basis, although the 'second generation' of universities established after Oxbridge, including Birmingham and Durham, are often described as 'red brick' - this is however due quite literally to the striking colour of bricks of the Birmingham University's Great Hall, among other buildings.
So it was those proud Castillian families avoiding intermarrying with Moorish and Jewish immigrants which originally gave rise to the term 'blue blooded' even though it has now spread far beyond the Iberian Peninsula. In these modern times, where we have moved on from a society straitened by notions of class, and certainly of racial purity, it is perhaps a phrase to be used sparingly, as it has the potential to offend any mixed race person who is fully aware of its origins.