I'm almost 60 and have been listening to and loving blues since I was a teen. I too long wondered what Johnny Cockaroo meant. Somewhere along the way I just assumed it was like a mojo, you know, a lucky charm or talisman of some type. A few years ago I was talking with a neighbor, an African American man in his 80s in our town of Austin Texas. He used the term Johnny Cockaroo and stunned me. I asked what he was referring to and he pointed to some flowers that I call Johnny Jump Ups. I looked up Johnny Jump Ups and found they have all sorts of medicinal uses going back centuries. In Shakespeare they show as Heartsease and can cause a person to fall madly in love.
I say, forget what else you've heard about John the Conqueror or mandrake. In Texas, for Black people, it's the Tri-color Viola commonly called Johnny Jump Ups by Whites.