The character in OBWAT was not meant to be Robert Johnson. The legend of Robert Johnson selling his soul is a legend within a legend. The character is named Tommy Johnson because that's who the original tale of the crossroads soul selling was based on.
Mike, to be more clear, I think what you're saying here is that Tommy Johnson is based on Tommy Johnson, another influential delta bluesman who used to use the sold his soul to the devil story to enhance his stage persona.
And in subsequent years, that story has been misattributed to Robert Johnson because they share the same last name and profession.