When you go to a blues jam, you should know some standards, you should know how to hear and play through common progressions, and you should know how to communicate with the rest of the musicians. The first two items on the list must be developed over time, but the final item, communicating with the band, can be learned quickly. There exists a pretty graceful nomenclature for laying out a road map for the band, and it's worth taking a few minutes to learn these terms and how they work together.
The first things you're going to want to communicate are the rhythm and the key of the song you want to play. Your basic choices for rhythm are as follows: Texas shuffle, Chicago shuffle, swing, 12/8, Latin, rock and roll, funk or slow. When you step up to the mic and turn to the band to let them know what you're looking for, here are a few samples of how to call songs, along with some examples of songs that fit the description:
Tempo is vitally important, and it is critical that everyone in the band has a clear understanding of the intended tempo. Don't be shy when you stomp it off. It might be a good idea to cut the time on the first bar of your two-bar count-off. Get the tempo straight in your head, and then in clear and authoritative voice, count it off: "One, two, one-two-three-four..."
A lot of blues songs have what is commonly referred to as a "Fast IV." What that means is that instead of riding on the first chord for four bars before going to the IV chord, you go to the IV on the second measure, then back down to the I for two more bars, then back up to the IV chord on bar five. Good examples of this are Bo Diddley's "Before You Accuse Me" and Robert Johnson's "Sweet Home Chicago."
Some blues songs start on the IV chord. In this case, it usually goes four bars on the IV, then four bars on the I, followed by two bars on the V and two bars on the I. Buddy Guy's "Mary Had a Little Lamb" fits this description.
There is also a tradition of the "Long I," in which the progression stays on the root tonality of the key for eight bars instead of four. BB King's "Better Lovin' Man" and "How Blue Can You Get," also by BB King follow this form.
Another common variation is the "Long V," in which rather than playing V-IV-I-I for the final four bars of the twelve bar progression, you sustain the V chord for an extra bar, making the final phrase, V-V-I-I. There are other such variations, and for the most part, they are easy to hear and feel coming if you are sitting in, and also easy to telegraph if you are leading.
If you want to lend a bit of arranging to your performance, you can embellish the intro considerably by starting from bar nine of the 12-bar progression. The typical shorthand for this is "From the V." When starting from the V, you essentially start the tune with the final four bars of the progression, V-IV-I-I. It is also quite common on the final (12th) bar to go back to the V.
Here's a typical call, incorporating several elements of this blues patois: "Before You Accuse Me, Chicago shuffle in A with a fast IV. Take it from the V, right about here (snap your tempo). One, a-two, a-one, two, three..."
So that's how you kick it off. Once the jam is on, if you are leading the tune, be bold. Pick the order of solos and communicate them unambiguously to the players. Don't feel like you're being bossy. Players infinitely prefer being directed to not knowing what's next and trying to decide by committee at the turnaround.
On that subject, maybe you need to look at your hands while you're playing. Keep playing and eventually you won't, but maybe you still do. At the very least, you need to pay attention at the turnaround. Maybe the leader will be wildly gesticulating for you to solo but you are so buried in your fretboard, you miss the cue. At bar eleven or so, pick your head up and watch for a transition from soloist to vocalist, from vocalist to soloist, or from one soloist to another.
The other important arrangement opportunity for you is the ending. Typically the most graceful ending is achieved by repeating the final four bars. Sometimes it is repeated once, sometimes twice. It is easily called for by a signal from the leader, very often an index finger twirled in the air in a sort of a "whoopee" gesture.
Your attentive band will already be looking for it if you've sung, cued solos and then returned to a final vocal verse. If they seem a little out-to-lunch, communicate with the bass player and the rest of the band will get dragged into your arrangement whether they like it or not!
There are several standard blues endings, and most of them you can direct, either by jumping in with confidence and getting the band to follow you or by gesticulation. Check out the ending of Ellington's Take the A Train for one standard up-tempo ending, and check out the Allman Brothers' One Way Out for a slow ending to an up-tempo tune.
There is a vast canon of material within the blues catalog, but there are some hundred or so songs that you run into pretty regularly. I have culled a list of 25 songs that are a good idea to have in your repertoire when attending a blues jam. Some are male vocal pieces, some are some are female vocal pieces, some are instrumentals and some are guitar oriented. The songs appropriate for female vocalist are denoted with an asterisk. Most songs are unisex, available and appropriate to male and female vocalists with a few pronoun changes, but some are better than others. You may notice that a lot of songs specifically oriented toward the female vocalist are more song-oriented and less based on common blues form.
That's about it! The blues are a deep and sacred music form, and a lot of people have a lot of themselves invested in it. Thinking that "It's only three chords" is a great way to become an average to bad blues musician. Thinking about economy of musical language, tone, phrasing, grace and restraint, and marveling at the beauty of a single perfectly placed note is a great way to become a fine blues player. Enjoy the music, enjoy your instrument, and we'll see you out at the jams!