House of Blues Essential (Box Set) would be a decent start for a broad intro to the Blues. Throw in some Stevie Ray Vaughan (eg Real Deal Vol 2), the Willie Dixon Chess Box (Dixon songs covered by a bunch of other artists - all classic blues songs), and maybe some Allman Bros, Susan Tedeschi, and you're covering a lot of ground. There's also a great acoustic blues CD called Hellhound on my Trail - the Songs of Robert Johnson (covered by others) that I really enjoy.
Hey, and don't forget - Cream, the Yardbirds, and Zep I of course. Being a rock fan you've probably already listened to a lot of blues covers with a heavier drumbeat.