Although I suppose it's not just the banjo's presence that is used for this signalling, but certain musical forms associated with Country (which I'm not sophisticated enough musically to know the origin of, tho it must trace back to blues in some fashion I'd wager).

I've been listening to a bit of early Country music lately and I find myself wondering how that music developed. Blues was obviously an influence, but I'd love to learn more about how exactly the artform developed especially given the racial dynamics at play and the fact that recording was in its infancy.