While eighth notes are typically straight in classical music (meaning that they are played exactly as written), jazz music typically applies a different "swinging" rhythm to the same notes. Some books often mistakenly mark the new rhythm as a dotted eighth note and a sixteenth note. However, this makes the second note too short. Rather, visualize the eighth notes as being a triplet with a quarter note as the first note and an eighth note as the second note. In addition, place the stress on the second note rather than the first one. In this way, the eighth note leads to the next strong beat. Applying this concept can take a great deal of practice, but listening to a great deal of jazz music and copying the way in which the musicians play rhythms is a good way to begin understanding "swung eighths."
The 12-bar blues typically consists of 12 bars in 4/4 time. In its simplest form, the chords used are the tonic (I), subdominant (IV) and dominant (V). In addition, these chords include a lowered seventh in their respective scales. So in the key of C, for example, the tonic chord includes a Bb, the subdominant includes an Eb and the dominant includes an F. When using these chords to accompany a jazz ensemble, it is best to place stress on the added seventh. You may even try placing the seventh in the bass. For example, while it could be tempting to voice a tonic chord with C, E, G and Bb, a better voicing would be C, Bb, E and G. Not only does the second voicing add a "jazz flavor" to your playing, it can also make some common jazz lines easier to play.
The common saying "less is more" is certainly true in jazz blues improvisation. Never feel like you need to be playing notes on every beat to construct a solo. In fact, letting the listener wait and build suspense as to what note you will play next can add tension and variety to your solo. At the same time, your solos should include patterns that will be able to be sung by the audience. To construct good solos, invent patterns that you like and practice them in every key. Commit them to memory so that you can use them whenever you need to do so. Improvisation requires a great deal of practice, but is quite easy once you gain a working knowledge of jazz chords and concepts.