Henry Gray, an heir to the Chicago blues piano throne, lives in Scotlandville, a small community on Highway 61 (the Blues Highway) north of Baton Rouge. As you approach his wooden bungalow, one of Gray’s granddaughters opens the screen door and shouts, “Grandma, there’s someone to see Grandpa,” sensing that a visitor carrying a notebook and a tape recorder could only be there to visit her famous grandfather.

The little girl’s mother invites the guest into the living room while Mrs. Gray go...