One of my colleagues young (13 years) son is a multi-faced musician
who, among his other interests, loves singing barbershop with the
local club. He drew his dad in as well. It seems to build character
and musicianship.

Barbershop is certainly distinctly American, and distinct from
'four square' British harmony. As such, it incorporates what
African Americans brought to the American music hall. However,
the citations brought out earlier strike me as one of those
attempts to credit the entire Industrial Revolution to the
inventor of the cotton gin.

Thirds and major and minor sevenths and diminished ninths did
actually exist in music theory even prior to the triangle trade.

Barbershop is a melding of traditions, all of which converged
in the burgeoning US and peaked in the 'gay 90s'.

No one tradition can claim it.

But it's great good fun for those who do it--- sort of like
singing shanties has become a century later.