Their objection isn’t “blacks” it’s “THE blacks” that’s raised their hackles.... this time.
You see there is no rational explanation of why the term “the blacks” is objectionable. Ask them to explain and you’ll get a lot of foot shuffling and phoney indignation as if of course it’s unecessary to explain.
He can refer to “the” blacks, whites, Irish, etc. Who the hell cares? It’s the word “the”.