group B vs blood group

group B and blood group both are nouns.

Word	Noun	Adjective	Verb	Adverb
group B	Yes	No	No	No
blood group	Yes	No	No	No

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

group B: the blood group whose red cells carry the B antigen
blood group: human blood cells (usually just the red blood cells) that have the same antigens

Other hypernyms of group B include blood type.

group B (noun)	blood group (noun)
the blood group whose red cells carry the B antigen	human blood cells (usually just the red blood cells) that have the same antigens