Under New York State's gradual abolition act of 1799, children of slave mothers were born to be eventually liberated but were held in indentured servitude until their mid-to-late twenties. Together with slaves freed by their masters after the Revolutionary War and escaped slaves, a significant free-black population gradually developed in Manhattan. Under such influential United States founders as Alexander Hamilton and John Jay, the New York Manumission Society worked for abolition and established the African Free School to educate black children. It was not until 1827 that slavery was completely abolished in the state, and free blacks struggled afterward with discrimination. New York interracial abolitionist activism continued; among its leaders were graduates of the African Free School. The city's black population reached more than 16,000 in 1840.
Is there an answer to this question (If it cannot be answered, say "unanswerable"): What borough was home to a notable population of free African-Americans?
Manhattan