Green’s Dictionary of Slang

(US) an area of New York City with a predominantly black population, covering those blocks between Tenth and Eleventh Avenues, between 59th Street and the low 60s.

1905		Palestine Dly Herald (TX) 19 July 5/3: Rioting between whites and blacks was resumed Monday night in the San Juan Hill district.
1910		Tacoma Times (WA) 8 July 2/2: A close friend of the black champion [...] promises that ‘San Juan Hill’, New York’s colored belt, will ‘agitate’ when [Jack] Johnson comes to town.
1919		Wash. Times (DC) 16 Sept. 7/6: One negro was killed, two others wounded, and a policeman was badly beaten in a race riot which broke out in the San Juan Hill district.
1921		Abbeville Press (SC) 3 June 7/2: If only the brush [sic], crass coloured people of San Juan Hill [...] would copy the manners and niceness of speech and feeling of their Southern brothers.
1946		Mezzrow & Wolfe Really the Blues 245: They’d both grown up in the ‘jungle’ of San Juan Hill (the tough neighborhood on 10th avenue along in the Sixties, just above Hell’s Kitchen).
1993		(con. 1900s–10s) I.L. Allen City in Sl. (1995) 239: During the hot spell of August 1900, about the time San Juan Hill was named, one of the city’s worst race riots [...] occurred in the Tenderloin [...] After the First World War, when the black migration to Harlem was well underway, San Juan Hill became known as Columbus Hill, denying its past.