When did the American police officers start wearing black instead of being the "Boys in Blue?"?

Um, my old department, the second largest in the US, wear blue, specifically robins egg blue, shirts. NYPD and LAPD , largest and third largest reps stably.wear a darker navy blue which unless in bright sunlight might look black to those with poor vision.. Many sheriff's departments, like LASD, the 4th largest department in the US wear Khaki
Houston Tx is the 5th largest department in the US and they wear light blue.

Perhaps you should get out more or, since you seem to care, e more observant.

No set uniform issue

Each department has their own colors

cops in my town used to be the boys in blue but have now adopted a paramilitary tactical Nazi looking uniform