If you say that someone has blue blood, you mean that they are from a family that has a high social rank. Rich people in the medieval Europe had so much wealth and power that they didn’t spend time outside in the burning sun. As a consequence, they were often so pale that their veins looked blue under their skin. This led to the popular belief that their blood was blue. Now it generally refers to people from old money: families who have been wealthy aristocrats for generations on generations.
Source: Urban Dictionary
