black rat vs Rattus rattus

black rat and Rattus rattus both are nouns.

Word	Noun	Adjective	Verb	Adverb
black rat	Yes	No	No	No
Rattus rattus	Yes	No	No	No

As nouns, black rat and Rattus rattus are synonyms defined as:

black rat and Rattus rattus: common household pest originally from Asia that has spread worldwide

Other synonyms of black rat include roof rat.

black rat (noun)	Rattus rattus (noun)
common household pest originally from Asia that has spread worldwide	common household pest originally from Asia that has spread worldwide