That's "Big Brother & The Holding Company's" Cheap Thrills above. That's a good white band blues album, and there's no question that janis is one of the most powerful rock performers of all time.
Zepplin 1 through IV are great. Sometimes it's best to start with more accessible rock/blues and work your way backwards. It might sound like rock, but Black Dog is all blues.
IMHO one of the best blues albums ever was buddy guy's Stone Crazy.
SRV was fantastic but his experimentation with crappy tone really dogged. I recommend the Sky Is Cryin album because it's got some of his most honest sounding work.
I didn't see anybody mention Jimi yet. Hendrix was arguably the most influential blues guitarist of all times. He really took the music to a whole new level, while still remaining pretty traditional. The Jimi Hendrix Blues record is a very good compilation featuring some shit that will stick in your head for the rest of your life.
Anything by John Lee Hooker is a must. You'll recognize a lot of it because it's been covered to death (all they play on the radio is george t version

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Elmore James was one of those guys who sounded exactly how you would want the blues to sound. Fantastic. Did a lot of work with buddy guy.
Grab an Allman Brothers compilation with tunes like Whippin Post and Midnight Rider.
Muddy Waters invented the electric guitar

If you listen to Muddy you'll recognize EVERYTHING. All muddy tunes have been covered and covered again because they're all absolutely great.
Eric Clapton's got some great stuff if you drive a Volvo and work at a bank. Barforama. (hey, what's a blues thread without some controversy)
One of my personal favorites is Howlin' Wolf. Grab something with Smokestack Lightin and Little Red Rooster on it.
Oh, and Albert King. This guy did more with one fucking note than most people do with an entire guitar. Very understated. Born Under a Bad Sign is his most recognizable tune...Drownin on Dry land is wonderful.
A lot of B.B. King's older stuff is awesome. He's still awesome. I've seen him twice....last time he could barely stand up, but he still put on a fantastic show. I can't listen to any of his newer records because honestly I think they suck, especially when he gets together with crappy radio-rock musicians...ugh.
Robert Johnson is the guy I hate to mention because it sounds so pretentious to say Robert Johnson. Really he was one of the first guys to pick out what became "traditional" blues...one of the first guys recorded too. There's a great Robert Johnson box set with all his recordings...get it. You'll hear sooooo many familiar lyrics you'll wonder if robert johnson maybe didn't write every song in the world.
Anyhow, once you really get into the blues, you'll hear Undertow in a whole new light!
Slackmaster 2000