A real good way to get the essence of the blues is to buy the Chess Records 50th Anniversary CD compilations. Look for them on Amazon. Put "Chess 50th" into a search and you will find them. If you don't know, Chess Records was a Chicago record label that recorded Blues musicians from the late 40's until the 1970's. If you can only buy one, it should be the Howlin' Wolf. If you can afford two, add Sonny Boy Williamson. After that, you'll need Muddy Waters (2 discs, the early years are better) and John Lee Hooker.
Every one of the Chess 50th collection is a gem and worth having. Buy as many of them as you can.
Cheers (from Chicago)