A prince of the blood is a legitimate male descendant of a country’s ruling monarch. The governess’s elevation of her young charges to the ranks of royalty reflects a subliminated anxiety regarding her own social position.
The idea of a pair of young princes enclosed in a towered building they cannot leave is reminiscent of Richard III's treatment of Edward V and Richard, Duke of York. After the death of their father, Edward IV, his brother Richard III was instated as the boys' legal protector, a role he fulfilled by having them incarcerated in the Tower of London and acting to prevent their succession. Both disappeared and, though no evidence was found, it was widely assumed they had been murdered. The governess's choice of simile suggests that, far from being the children's protector, she is actually their jailer - and perhaps worse.