'blue-blooded' originates from the Spanish term 'sangre azul', where 'sangre' meant 'blood' and 'azul' meant 'blue'.

'sangre azul' was used to describe the pale skin of Spanish nobility, which made their veins appear blue, and eventually became the English term 'blue-blooded'.

Initially, it referred to the visible veins of nobility, but over time it evolved into its current meaning of 'noble or aristocratic descent'.