There are several explanations for Johnson's miraculous musical improvement.
1.The Devil/Legba: Johnson displayed uncanny growth in musical skill in a very short amount of time. He had only left Robinsonville for just short of two years(1931-33) and had returned a master of songwriting and the guitar. Rumors began to spread that Johnson had traveled to the crossroads in Mississippi, and in a Faustian sense, had sold his soul to the Devil for musical talent. His songs seemed to lend truth to the rumors with their frequent hellish references to the devil, hell and the crossroads. The theory was never substantiated by Robert Johnson, unlike his contemporary Tommy Johnson, who explicitly claimed to have signed a deal withe the devil.
He also makes references to hoodoo in his songs, such as "hot foot powder"(Hellhound on My Trail), a woman's "nation sack"(Come on in My Kitchen) and a "mojo bag"(Little Queen of Spades). Hoodoo was a religious practice of African Americans in the Deep South. Its rituals could be used to acquire skills or knowledge from Legba or other African deities believed to visit the crossroads, but they do not require one to deal one's soul to a god. It is suspected that he may have been a practitioner of hoodoo, using the rituals to acquire his musical skills. The Devil is often falsely equated with Legba in literature, but the Euro-American Devil seems to embody not just the traits of the trickster-god Legba, but also the sum of traits of other African deities. He could have tried to perform a ritual to any of these deities, including, but not limited to Legba. Acquiring his skills from any of the hoodoo gods could have easily been construed as striking a deal with Satan himself.
2.Practice/Learning: This is a more rational theory. His lessons with Ike Zinnerman are often overlooked by those looking to pin a pact with the devil on Robert Johnson. To what extent Zinnerman actually improved Johnson's play is very difficult to determine. Other local influences were enormously beneficial to Johnson's development. His brother Charles introduced him to the guitar. He watched and listened to Son House, Willie Brown and Charley Patton perform in Mississippi. Joseph Monzo claims that he was also exposed to a large variety of recordings, including Duke Ellington. He could have spent the time in Hazelhurst learning to integrate the styles of his predecessors into his own style.