There are the old blues like Robert Johnsone, Muddy and Howling wolf were later [ 60s ] , or theres jazz like Jelly Roll Morton ,Jug bands or you could listen top the people that Elvis copied like Big Joe Turner .
If you go down the blues root Lightning Hopkins is good , or Sleepy John Estes ,Son House or Leadbelly .And no they don’t all sound the same , there’s a rich variety .