willow bell vs bellflower

willow bell and bellflower both are nouns.

Word	Noun	Adjective	Verb	Adverb
willow bell	Yes	No	No	No
bellflower	Yes	No	No	No

As nouns, bellflower is a hypernym of willow bell; that is, bellflower is a word with a broader meaning than willow bell:

willow bell: perennial European bellflower with racemose white or blue flowers
bellflower: any of various plants of the genus Campanula having blue or white bell-shaped flowers

Other hypernyms of willow bell include campanula.

willow bell (noun)	bellflower (noun)
perennial European bellflower with racemose white or blue flowers	any of various plants of the genus Campanula having blue or white bell-shaped flowers