I'm a fan of both old-time and blues idioms. I think a lot of the "blues" recorded by hillbilly musicians reflected more an attempt to ride on the coattails of the popularity of blues and don't sound authentic to blues buffs. Although Blind Lemon Jefferson's records and tours influenced the Carters to take his tune from One Kind Favor for "Today has been a lonesome day (or whatever it's called)."   It think it's crucial as Joseph pointed out, to separate out the music before Jimmy Rodgers and after, as lots of folks put "Blue" in their titles and the giveaway yodels after that success... I don't know of any black blues recordings that incorporated yodeling but I guess that would be another topic.