Bill Cork has an interesting piece up on the response in the United States to Pope Gregory XVI's denunciation of the slave trade, including the less than imitable example of the remarkable first Bishop of Charleston, John England. (One of his successors, Patrick Lynch, was a slave owner and a staunch defender of the Confederacy.) When Popes speak « Built on a Rock It's hard to imagine that something spelled out so clearly by a Pontiff could be so soft pedalled by an American bishop today, when communication is so much improved.
Incidentally, this was also the time that Isaac Hecker was growing up, and exploring the Transcendentalists (if I have my dates correct), and a few years before he embraced Catholicism. As far as I could tell (my copy of "The Paulist Vocation" is packed away somewhere in DC right now), he didn't have much to say about slavery. As a New York lad, I guess, it didn't direclty impinge upon him.
A sobering read on the issue is John Noonan's "A Church that can and cannot change." [Do also read Cardinal Dulles's review in First Things.]