blue blood vs princess

blue blood and princess both are nouns.

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

As nouns, princess is a hyponym of blue blood; that is, princess is a word with a more specific, narrower meaning than blue blood:

blue blood: a member of the aristocracy
princess: a female member of a royal family other than the queen (especially the daughter of a sovereign)

blue blood (noun)	princess (noun)
a member of the aristocracy	a female member of a royal family other than the queen (especially the daughter of a sovereign)