black rat snake vs rat snake

black rat snake and rat snake both are nouns.

Word	Noun	Adjective	Verb	Adverb
black rat snake	Yes	No	No	No
rat snake	Yes	No	No	No

As nouns, rat snake is a hypernym of black rat snake; that is, rat snake is a word with a broader meaning than black rat snake:

black rat snake: large harmless shiny black North American snake
rat snake: any of various nonvenomous rodent-eating snakes of North America and Asia

black rat snake (noun)	rat snake (noun)
large harmless shiny black North American snake	any of various nonvenomous rodent-eating snakes of North America and Asia