Lighter, I think your conclusions about shanties created in or coming from the slave trade pretty well sum up what I've been able to (not) find on this subject. Even the references to the "Congo River " version of "Blow Boys Blow" are somewhat scarce. I did find this collection by Robert Frothingham, called SONGS OF THE SEA AND SAILORS' CHANTEYS: AN ANTHOLOGY, from 1924:

http://books.google.com/books?id=owcoe5PU6WcC&pg=PA244&dq=%22Blow+Boys+Blow%22&lr=&cd=1#v=onepage&q=%22Blow%20Boys%20Blow%22&f=f

But he substitutes "Old shot and shell, she breaks the embargo," for the line about "black sheep". (p. 245). I also found this single interesting verse in IN GREAT WATERS: THE STORY OF THE PORTUGUESE FISHERMEN, by Josef Berger (originally printed in 1941), p. 43:

http://books.google.com/books?id=ZC4diHs-RWMC&pg=PA43&dq=%22Blow+Boys+Blow%22&lr=&cd=149#v=onepage&q=%22Blow%20Boys%20Blow%22&f=

The author seems to be discussing the New Bedford whaling fleet and how some of them turned to the slave trade, and he presents this verse from "an old whaling chantey". Unfortunately, he gives neither a date nor a source for this.

There seems to have been a fluid situation among some of the whalers, who took up slaving, and then turned to piracy, all in the thirty or forty years before the Civil War. As with so many cases, one would like to know what they were singing as they passed through these various career changes!