If you can get smooth with the basic blues shuffle, you can keep most people happy with approximations of SRV's "Cold Shot" and "Pride'n'Joy".

If you're confident that you can do some classy solo work, you can add some slow material like Albert King's "Blues At Sunrise". Realisticly, you can only plan on one slow song per set (none in the first set). People come out to have a good time, and blues are actually fairly upbeat music. The lyrics can be about pain and misery, but the tunes should have a smooth groove.

Stuff like "Caledonia" goes over well, 'cause its familiar. I recommend an old Canadian band called the Downchild Blues Band. If they can't show how its done, you're just not payin' attention! :D

Newer material, like "Blue On Black" from Kenny Wayne Shepherd is sure-fire (and its easy to play!!)

While you're starting out, it helps to remember that almost everyone in the audience will give a tune 9 out of 10 if it has a good beat, and its easy to dance to. The remaining few, who know a thing or two about music, will be OK if they can see that you really are trying to put on a good show.

Best of Luck!