prayer vs blessing

prayer and blessing both are nouns.

Word	Noun	Adjective	Verb	Adverb
prayer	Yes	No	No	No
blessing	Yes	No	No	No

As nouns, blessing is a hyponym of prayer; that is, blessing is a word with a more specific, narrower meaning than prayer:

prayer: the act of communicating with a deity (especially as a petition or in adoration or contrition or thanksgiving)
blessing: the act of praying for divine protection

Other hyponyms of prayer include devotion, benediction.

As nouns, blessing is a hyponym of prayer; that is, blessing is a word with a more specific, narrower meaning than prayer:

prayer (noun)	blessing (noun)
the act of communicating with a deity (especially as a petition or in adoration or contrition or thanksgiving)	the act of praying for divine protection
earnest or urgent request	the formal act of approving
a fixed text used in praying	a ceremonial prayer invoking divine protection
reverent petition to a deity	a short prayer of thanks before a meal
someone who prays to God	a desirable state