Watch the Become a Kung Fu Master of Rhythmic Feels and Form online guitar lesson by Jeff Scheetz from Blues Jam Survival Guide
With blues jams it is important to know the basic blues feels, like 12/8 or slow blues, versus a shuffle feel, or a funk feel. You need to know the basic blues progressions, the 12 bar, the quick 4, starting on the 5 etc. These things will get you further in blues jam environments than learning specific songs like you would have to do if you were doing a rock jam. So work on the forms, learn some turnarounds, some licks, some chord variations, and you will be ready when someone calls you up onstage and then says "Blues in A".