Highness vs blue blood

Highness and blue blood both are nouns.

Word	Noun	Adjective	Verb	Adverb
Highness	Yes	No	No	No
blue blood	Yes	No	No	No

As nouns, blue blood is a hypernym of Highness; that is, blue blood is a word with a broader meaning than Highness:

Highness: (Your Highness or His Highness or Her Highness) title used to address a royal person
blue blood: a member of the aristocracy

Other hypernyms of Highness include aristocrat, patrician.

Highness (noun)	blue blood (noun)
(Your Highness or His Highness or Her Highness) title used to address a royal person	a member of the aristocracy