The California Gold Rush saw people from all races, genders, and ages flock to the area. It was difficult to maintain order in a lot of these new communities being set up. Adding to this scene were northern gamblers, failed gold miners, outlaws, and prostitutes who followed the money south or were simply driven out of San Francisco by the local vigilante committees.

One block in particular with block-row houses and businesses was called Calle de los Negros among those who spoke Spanish, or “Ni**er Alley.” It graphically appeared on official city maps in the early American era.

The city in the early 1850s had a population of nearly 4,000. It was considered the toughest and most violent and lawless cities west of Santa Fe. Most men walked the streets armed with pistols and knives, and it was said that a murder occurred every day.

An explanation of how the street got its name was found in a letter to the editor in the March 24, 1877 edition of the Los Angeles Express. The street was unnamed at first, but it “owned by some of the most substantial citizens of Los Angeles” who were “men of very dark complexion.” The letter went on to explain that one morning, “when the people arose, they found a placard put up at each end of the alley bearing the words Calle de los Negros,” which they interpreted as a sign of reproach. The letter concluded by explaining that the residents of the street traced the authorship of the insulting placards to prominent ranchero, soldier and politician José Antonio Carrillo (1802-1862), who apparently had an ongoing feud with residents on the unnamed street. The area is where the Los Angeles Chinese massacre took place in October 1871.  After the Chinese Massacre Los Angeles politicians took action to clean up the lawless area and renamed “Nigger Alley” North Los Angeles Street.

sources:

https://www.kcet.org/shows/lost-la/calle-de-los-negros-las-forgotten-street