The blue in "blue blood" could have come from silver

Historians suspect that argyria could have been the origin of the term “blue blood” is used to describe people of noble birth. In earlier times, it was common for European nobillity to have skin that appeared to have a blue or greenish cast especially made obvious by their pale skin that were "untouched by the sun" - - another mark of status among the wealthy who didn't have to do labor outdoors.