infant vs blue baby

infant and blue baby both are nouns.

Word	Noun	Adjective	Verb	Adverb
infant	Yes	No	No	No
blue baby	Yes	No	No	No

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

infant: a very young child (birth to 1 year) who has not yet begun to walk or talk
blue baby: an infant born with a bluish color; usually has a defective heart

infant (noun)	blue baby (noun)
a very young child (birth to 1 year) who has not yet begun to walk or talk	an infant born with a bluish color; usually has a defective heart