Sunshine of your Love (Cream)
Although it doesn't feel exactly like a traditional blues, Cream's "Sunshine of Your Love" is actually a pretty good place to start in learning the blues. The main song riff is based directly on the D blues scale, so it should provide a good basis for learning that scale.
Hideaway (John Mayall & the Bluesbreakers)
This is a perfect blues to learn for playing with other guitarist - it's an instrumental with a really memorable theme, so you don't need to worry about singers and lyrics. The tab and audio links here are for the John Mayall & the Bluesbreakers version of the song featuring Eric Clapton on lead guitar, which is great, but I really highly recommend hunting down the original version by Freddie King (listen to the audio on Spotify). Freddie opts to play the main lead guitar theme using open strings, which frankly sounds much better to me.
Killing Floor (Howling Wolf)
This is great! The lead guitar part here (played by the late great Hubert Sumlin) uses "sixths" - intervals that give the guitar part a real meaty sound. If you swear you've heard the sound of that lead guitar part before, think of the intro to the R&B classic "Soul Man". Once you've learned the riff, be sure you can play it in other keys, so you can work it into different blues songs you're playing.
Pride and Joy (Stevie Ray Vaughan)
There is so much to learn here - the great leads, the simultaneous rhythm/lead work, etc. that it can be overwhelming. To get started, concentrate not on all the great notes he's playing, but the feel he's imparting... despite the dazzling guitar work, it's very "relaxed". Pay attention in particular to his great rhythm playing when the vocals come in at around 0:30... Stevie is muting strings on his downstrokes, and allowing them to ring briefly before muting them again on the upstrokes. Look at the tab to find the notes he's playing during this section, and try playing through the entire twelve-bar blues using this rhythmic pattern.
I Believe I'll Dust My Broom (Robert Johnson)
This Robert Johnson classic does a perfect job of outlining the use of the two-string blues shuffle, moving from the E5 to E6 chord and so on. "Believe I'll Dust My Broom" also starts with one of the quintessential blues turnarounds - learn this, and be able to play this in different keys. You can use that lick both at the start of a tune, and in the "turnaround" at the end of the 12-bar blues form between verses.
Boom Boom (John Lee Hooker)
Call-and-response is one of the tenets of the blues - and an essential component of blues guitar. The idea is the guitar plays a theme or some sort of lick, then the band responds. John Lee Hooker's "Boom Boom" demonstrates this beautifully - at the beginning of the song, the guitar trades call-and-response with the band. Then John Lee takes over call-and-response duties for the guitar once the verse begins. Try using this concept in your band - it's a lot of fun!
The Thrill is Gone (B.B. King)
Space. Anyone who has listened to B.B. King knows his guitar work revolves more around the space between the notes than the notes themselves. You can hear that in "The Thrill is Gone" - B.B. isn't playing many notes, he's just playing a few and making them count. Experiment with this philosophy, and try cutting out about 75% of the notes you usually play. The real trick is to make sure the notes that remain are the right notes! The song is also notable for being a minor blues, which contains a slightly different chord progression.
Mannish Boy (Muddy Waters)
You'll know this riff the moment you hear it... this Muddy Waters tune contains one of those "must-know" guitar riffs. This is a song that will take you approximately 30 seconds to learn (there is only one chord!) but a lifetime to emulate. Like most of Muddy Waters' work, it perfectly encapsulates that late-night, raunchy, raw Chicago blues sound.
Red House (Jimi Hendrix)
Not surprisingly coming from Jimi Hendrix, there is a whole lot going on here, and you could spend weeks working through this one song. Here are a couple things to focus on - the chords Jimi uses right off the bat in the intro (an open d7 shape slid up the neck turning it into a Bb7, and his great use of call-and-response with his voice in the verse.