Great article here – supported by copious recordings – by Seth Shumate. One In Every Pocket: The Rise of Harmonica on Old Time Music.
The discipline of old-time harmonica is as distinct aurally as it is in technique from post and even pre-war blues. But they’re all tongue-blocking styles (no wonder I bang on about tongue-blocking so much).
Here’s an interview with Seth on Old Time Central. He does Skype lessons too if you fancy a bit of Old Time one on one.
I was alerted to Seth and his music last year by Ed Hopwood, who also introduced me to one of my favourite Old Time tunes Ham Beats All Meat by Dr Humphrey Bates & His Possum Hunters.