1. When you get to the jam, find the coordinator, introduce yourself and signup to play. Let them know what instrument you play so that they can begin to think about whom they will pair you with. Then, while you are waiting, listen to some great music and mingle with other musicians.
2. The singer is the bandleader on stage. That means they call the tune, signal the breaks and the solos. If you sing, it's crucial that you know how to do this. The best way to learn is to watch others and listen. When in doubt, ask for help from someone that's done this before.
3. A solo is your opportunity to shine. But it can go well, or it can go poorly. In general, try and play something that fits the feel of the song. Metallica riffs doesn't go well with a blues tune. Look at the vocalist/leader during your solo. They may cut you short and you want to see it coming. If you aren't paying attention, you'll miss the signal and step on the next guy. Finally, if you are not soloing, bring the volume down so they don't have to turn up so much to be heard.
4. Listen. Learn. Have fun. A blues jam is a great learning experience. But you get out of it what you are willing to learn. Don't go in with any preconceived notions. Ask a lot of questions. And listen to the guys that have been doing this a while.
5. Be musical. That means ... listen to what's happening up there on stage. Don't just listen to your one part. Listen to the whole mix. The whole point of getting on stage is to provide entertainment. It's about the music, not some selfish goal. And you have to understand your part in the arrangement. This comes, first and foremost, from listening to the music you are trying to play. If you come out to the jam and you don't know the first thing about blues, then it's probably best if you listen for a while before getting up there. There are those that get it, and those that don't.
6. Know some blues tunes. The way to earn the scorn of the rest of the musicians is to show up and call out some classic rock or other non-blues tune that's unfamiliar to them.