CHICAGO — The audience of Black journalists was prepared for a combative exchange well before Donald Trump took the stage Wednesday for an interview at their annual gathering in Chicago. Yet when Trump, just minutes in, began questioning Vice President Kamala Harris’ racial identity, there was an instant ripple of reaction — a low rumble that grew into a roar of disapproval. Sign up for The Morning newsletter from the New York Times “I didn’t know she was Black until a number of years ago when s