When did Philadelphia become the unofficial Black America?

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American Colonization Society and the Founding of Liberia | Encyclopedia.com

American Colonization Society and the Founding of LiberiaBLACK COLONIZATION: FROM TALK TO ACTIONCOLONIZATION SOCIETIES: NATIONAL, STATE, AND LOCALCONTRADICTORY CONCEPTIONS OF BLACK CAPABILITIESCOLONIZATION PROGRAM BACKFIRESHOLDING FAST TO COLONIZATION RATIONALESON THE GROUND IN...

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Two of the most powerful were founded in New York City and Philadelphia, the latter the informal “capital” of free black America.

I wonder how that came to be.