What is it about the Blues?

What is it about the Blues that makes it such an influential genre.
Listening to the Rolling Stones’ Shine A Light album, I was awed by the continous referencing to the Blues. Eric Clapton sounds best when he plays the blues. Even a wannabe band in Pune wants the Blues tag.
What did the Blues have? It was dark, depression, escapist music that offered no hope. The music was the escape and that was it. Elvis was soaked in the Blues. Now I listen to Amy Winehouse and I get the Blues. What is it about the Blues? Anybody?