alpha blocker vs blocking agent

alpha blocker and blocking agent both are nouns.

Word	Noun	Adjective	Verb	Adverb
alpha blocker	Yes	No	No	No
blocking agent	Yes	No	No	No

As nouns, blocking agent is a hypernym of alpha blocker; that is, blocking agent is a word with a broader meaning than alpha blocker:

alpha blocker: any of various drugs that block alpha-adrenergic receptors; used in treating benign prostatic hyperplasia; relaxes the muscles of the prostate and bladder
blocking agent: a class of drugs that inhibit (block) some biological process

Other hypernyms of alpha blocker include blocker.

alpha blocker (noun)	blocking agent (noun)
any of various drugs that block alpha-adrenergic receptors; used in treating benign prostatic hyperplasia; relaxes the muscles of the prostate and bladder	a class of drugs that inhibit (block) some biological process