"Royal blood" simply means being a direct descendant of a royal family, typically through a lineage that can be traced back to a monarch. This means that you are related to someone who held a position of power and authority within a monarchy. The further back your connection to royalty, the less likely you are to be considered "royal."
For example, if your great-grandmother was a princess, you could be considered to have royal blood, but you wouldn't be considered royalty. This is because royalty is often defined by those who currently hold a position of power within a monarchy.
The concept of royal blood has historical significance, as it was often used to justify the power and authority of monarchs. It was also used to maintain a sense of exclusivity and privilege within a royal family, with marriages typically arranged within the family or among other noble families to maintain pure bloodlines.
The term "royal blood" is often used metaphorically to describe someone who has a certain sense of power, privilege, or nobility. However, in most cases, it simply refers to a person's lineage and their connection to a royal family.
