What is the origin of the idiom "blue blood" and when to use it?

The origin of the idiom "blue blood" can be traced back to medieval Europe. The phrase originated from the belief that the nobility had a fairer or paler complexion, which made their veins appear more prominent and bluer in color. This physical characteristic was attributed to their higher social standing and noble lineage. The term "blue blood" was used to distinguish the nobility from the commoners, who were believed to have less visible veins and a redder complexion. This phrase is commonly used when referring to individuals who come from families with a long-standing history of wealth, privilege, and influence. It can be seen in conversations about royalty, aristocrats, and prominent families with generational wealth.