Disclaimer, i know my blues playing job as a bassist.
To further Mambo4s excellent post, let me say that structure to build on is the key.
If you just jam away making up lines you fail to understand your role in the big scheme of things. You as the bassist, like the drummer in Blues, are there to be a foundation and a structure that allows others to jam away making up lines.
If you are with players of the highest calibre of Blues understanding then you will be able to have some liberty with what you do because these players are qualified enough to recognise what you are doing and make the most of it.
Point 1 about Blues jams is that they have a short time for any player to show their worth, so the lead player will cram all his licks and chops into that time to prove his worth to the determent of the song and the others around him.
Point 2 Blues is about the words, its message is in being able to hear the story being told, not the music or its format. Again this is where songs at jam session fail because the players tend to forget the message and concentrate on thier playing.
Point 3 In a jam situation no-one can be stringer than the weakest player on stage, so as he cannot come up to the higher level, everyone come down to him..even if it is way below you skill base. By exposing the weak link you expose the song to fail.
Point 4 Blues has a history and a respect of what has gone before needs to be held up for it to go forward. This is in how it is played, not what it is played on. If the playing moves out of the what defines blues then it stops becoming Blues, you in essence change it into something else. This is part of the challange, to find new ways of expessing it will keeping it in the spirit of all before.
Even though all i have said applies to jams, it is at the jams you learn these points to take to your own music and playing.
As a rhythm section if you lay the foundation and outline the structure, you can allow the players around you to play and improvise. In a band situation their playing will work out what works, what does not, what sounds good, and what sound great...they will then settle on playing this way everytime. But if your rhythm section is jamming everytime, then every song will sound and feel the same because no one has the control.
That said it does not mean you cannot change what you do, once this settled period is in you can make subtles changes and develop it further, but it is developing within the spirit of the song. As it is now an established song you have to be true to its spirit, if you have a following and the song is a favourite, then you really do not want to change it from what fans identify it as. The bass line to many songs i play today have evolved on from the ones i recorded in the studio.
So as i have witnessed and to my experience solid structure, that allows you to set up lead players while supporting the the players around you ensures the music and therefore the song and therefore the performance is good.
To this end i have a vast library of structures to call from which i use with any drummer in any playing situations i find myself in that to the layman may sound like we are jamming, but to the players let them know what they shouldn't do rather than what the can do.
Blues if we look at it has many many layers that can be exposed and played to if players have the patience and knowlege to expose them, rather than burying them under the weight of cliche.
For example if you want to help your lines swing better, then listen to Count Bassie and the Duke Ellington Big Bands. They are playing to a foundation and a structure everytime, but this lets the playing shine through and no one can say they are going through the motions because each performance is so on it.
