I was just mentioning to a friend the tremendous version of Give Me One Reason by Tracy Chapman (I also like the version by Kandee). The woman I was talking to didn't know what I mean when I said that it was real blues. She didn't know there was a difference between real blues and R&B or bluesy rock. The difference is that real blues has a specific harmonic and lyrical structure to it. She didn't know what I was talking about until I said, "It's like haiku; you have to follow a particular pattern." (of course that's only the word structure, there is also a chord structure). She knew what haiku is but not blues. Why do schools teach haiku but not blues? Haiku is stupid. Blues is some of the best entertainment ever invented.
Here are some true blue songs that you may not have known are blues. Mustang Sally is often played by R&B bands, but it's true blues. Elvis doesn't do a great job of bring out the blues sound in Hound Dog, but the chord and lyrical structure fit. Stormy Monday has been covered by a lot of artists who don't do strictly blues. Santana and Fleetwood Mac both did versions of the blues song, Black Magic Woman.