This article for the Paranormal Corner was written by Jose Prado.
To further the lie they then confused Robert Johnson with his friend Tommy Johnson. Why would American historians confuse them? And why would they say either one of them sold their souls to the Devil? These are the reasons why.
1) America has a long history of racism and ignorance of other ethnic groups and cultures and more than likely white Americans in the 1920’s couldn't tell the difference between one black musician named Johnson and another.
2) Both Johnsons were Vodusants, that is to say, Voodooists, their religion was Voodoo, they believed in different spirit forces that ruled all things. 
But to white society in 1920's America it was all 'pagan' blasphemy mixed in with 'devil worship', they couldn't tell the difference.
3) They were both Hoodoomen (Conjuremen, Conjure workers, Hoodooists, Rootworkers etc) they both practiced Hoodoo, the witchcraft of the Voodoo religion.
Contrary to popular belief, not all people who believe in Voodoo are witches. A Voodoo witch practices Hoodoo and is referred to by the above names. Voodoo is the religion, the stories of the Spirits/Gods/Divinities the prayers etc...Just as not all Catholics are priests or can bless things. Or not all Wiccans are witches. There is a clear line between a regular Vodusant and a Rootworker. But to white supremacist America, no one saw the difference. 
Many today still view it as Satanism. But these men, back then both openly admitted to practicing witchcraft. The controversy they stirred up must have been enough to have white Americans talking about ‘demons’ stirred up by blacks under every corner. Or then again, it may not have been that controversial since many white Americans already had such racist notions to begin with. If anything all it did was solidify those notions. With Tommy Johnson openly speaking of his pact to a Pagan God or Spirit? It was propaganda that racist America could use to spread to one another about the ‘heathen’ blacks. To them it was clear: a black blues musician sold his soul to Satan. Clearly to them a spirit or God from another religion is 'evil' and of Hell. And since Robert died of seemingly un-natural causes, it was said his death was due to the Faustian deal with Old Scratch (nickname for the Devil in New England and Pre-civil war America) who had come for his due.
4) Both Tommy and Robert were known Hoodoomen. In fact you'll even find Occult messages in Robert's lyrics about how his lover used Hoodoo to cheat at Gambling along with other hidden messages.
And since he wrote a song about Hell Hounds, to them it was proof he was consorting with Satan. Of course for all anyone knows, the song was a reference to something else. We know his song Crossroads was about hitch hiking on the Crossroads. But most people think it was referencing his non-existent 'deal'.
5) Both Tommy and Robert were friends, and both were Vodusants and Hoodoomen. So when one person talked about one of them it was probably inevitable the other would come up in conversation. So that may have started the confusion for white historians in the first place.
But despite popular legends and confusion, the facts are these:
* Robert Johnson did not make any deal on the Crossroads (that we know about, it would not have been odd for any Vodusant to make one with a spirit there)
* The one who made the deal was Tommy Johnson his friend.
* Tommy did not sell his soul to any spirit, let alone to Satan.
* Tommy Johnson made a pact (an unbreakable promise to a divine being) with the Vodoun Crossroads spirit (Or God as I believe him to be) Papa Legba to learn how to play the guitar.
* What he promised is unknown to us; pacts are sometimes secret and are between the spirit or Deity and the human.
* Whatever he promised, we can be sure of one thing: it was not his soul he gave away. Legba is a benevolent Divinity. Good spirits or Deities do not do things like that.
* Pacts could be promises of leaving offerings to a spirit or God when you have money, paying them tribute on special days like their feast day, wearing special colors, beads, or amulets or other things to signify your allegiance to them etc... Pacts are also not exclusive to non-christian religions. Plenty of pacts can be found in the Bible. Noah’s pact with God (Genesis 9:11-17) or God’s pact with Abraham (Genesis 15:1-4). Vows of chastity made by Christians are pacts with God. A pact is a solemn promise to a Deity or spirit that in exchange for a gift they will bestow upon you, you will reward them with something in return. A gift for a gift in other words.
* Note: In some beliefs and stories it is Satan who is summoned on the Crossroads. So it's not impossible. I have met Hoodoo men/women who perverted the teachings of Hoodoo and summoned demons under Satan, and they did go to Crossroads or Cemeteries to make deals with them. But they do these things "under the table" or in secret. So don't expect people to openly say they are doing these things. Because they would be shunned as blasphemers by the rest of the community. No one would be dumb enough to openly admit they were doing that.