The term 'blue blood' or 'blue blooded' derives from pale (/white) skin. Where you can see the dark blood flow through the veins. That looks rather blue than red.

It's first mentioned in ancient Spain, where the ruling class (the aristocrats) came from the north. Those had lighter skin than the local people.

Later 'blue blood' has been proofed again and again, in all countries acoss Europe. Because the upper class usually wasn't as exposed to the sun (doing manual labor on the fields) as they are today: Sitting on the beach and being photographed all day.