I've been racking my brains trying to think of a largish introductory blues anthology, and I just can't. There are plenty of cheapo labels about though and I would particularly recommend Indigo and Catfish if they are still in existence.

Otherwise, a good way of getting to grips with the idiom is to listen to The Dick Spottswood Show on Station WAMU, Washington DC. You can pick it anytime via the station's Listen Again facility on http://www.wamu.org/

The programme covers the whole gamut of race, country and ethnic music which was issued on 78 records from the 1920s through to the late 1950s. So there isn't actually all that much blues on it. However, Spottswood is a world renowned authority in this area and it's the best way I know of understanding the blues in relation to all the other music that was being played and recorded in the American south.

Cheers,

Fred McCormick