Kat's idea is a good one. A little known fact is that some slaves were hired aboard cargo vessels by their masters. Other crewmen included free blacks. Most of this was on river traffic. I would think that over time, some of these slaves escaped.

Here is perhaps the oldest Negro song that talks of freedom, by a former slave who was alive at the time of the Nat Turner insurrection. The song was collected by Lydia M. Child and published in 1836.

Lyr. Add: A FEW MORE BEATINGS

A few more beatings of the wind and rain,
Ere the winter will be over-
Glory, Hallelujah!
Some friends has gone before me,-
I must try to go and meet them-
Glory, Hallelujah!
A few more risings and settings of the sun,
Ere the winter will be over-
Glory, Hallelujah!
There's a better day a coming-
There's a better day a coming-
Oh, Glory, Hallelujah!

Child wrote of the singer, Charity Bower: "With a very arch expression, she looked up as she concluded and said, "They wouldn't let us sing that. They thought we was going to RISE, because we sung better days are coming."
Child, Lydia Maria, "An Appeal in Favor of that Class of Americans called Africans." New York, J. S. Taylor, 1836.