I would argue that the origin of the blues goes further back than the 1920's. Scorcese's documentary probably chose that period as a reference point, not meaning to imply that before the 20's there was nothing resembling the blues. If so, then he's hawking the equivalent of a Creationist explanation.

My own musical knowledge of the origins of the blues only goes back as far as the 'field hollers' of the slaves that people like Leadbelly so faithfully recreated. To me there are blues elements in those hollers. That and the Negro church hymnals of the pre-civil war period, in which the structure of repeating the first line of a verse twice I've heard explained as the congregation, most of whom were illiterate, repeating back the line that was sung to them by the songleader:

I got a woman, she sure don't treat me right,
I got a woman, she sure don't treat me right,
Stay out drinkin' and don't come home till daylight.

(Of course, that wasn't what they were singing in church, but the example was offered only as an illustration)...whomever it was doing the research here on church hymnals, I wonder if s/he came across any evidence to substantiate this?

And in keeping with an evolutionary explanation, the origins probably go back further than that, and with the passage of time and mutations across generations, the blues has evolved into what we hear today.