Harmonica production was too low in the 1860s for harmonicas to have
been very widespread in the US during slavery times. Production
wasn't
really large enough until about the 1870s or '80s, which was in
fact a
time of great hope for black folks, with African Americans actually
getting elected to State office, until white racists figured out ways
to shut them out of the entire democratic process for the better part
of a century.
The popular image of the harmonica being played by Civil War soldiers
was created by Hollywood screenwriters. They remembered the very real
phenomenon of the harmonica being very widespread during the first
world war of 1914-1918, when annual harmonica production (and
exportation) was well into the multiple millions, and projected that
back to the Civil war scenes they were writing (soldiers gone off to
war, a past era, harmonica). Some folks I know will disagree, but I'm
pretty certain this is a false image. The production and distribution
just weren't there.