Minor Blues are a variation of the twelve bar blues, but instead of being in a major key, they are (can you guess it?) in a minor key. The basic form of the minor blues is the following:

|| Im | Im | Im | Im | IVm | IVm | Im | Im | V7 | V7 | Im | Im ||

where Im is any minor chord, IVm is a minor chord a fourth above it and V7 is a dominant chord a fifth above it. For example, in the key of F minor, a minor blues progression will be:

|| Fm | Fm | Fm | Fm | Bbm | Bbm | Fm | Fm | C7 | C7 | Fm | Fm ||

Unlike regular blues (twelve bar blues, that is), minor blues are not often found in rock or blues music, but are mostly found in jazz. This is because rock and blues music usually rely on formulae that work, while jazz musicians are always looking for new boundaries to break, and what is more obvious than taking a perfectly good major progression and making it minor? (Incidentally, I'm not dissing rock or blues in any way. If you disagree with me, /msg me and I'll kick your lights out. Just kidding. I play rock, blues and jazz, and that is my take on matters.)

You will notice that not all the chords are minor. The fifth is dominant. This is because a minor fifth will simply sound totally lame. The dominant fifth is derived from the melodic minor scale, and thus has a major third. This, incidentally, is done a lot in music. The fifth mode of a minor scale is minor too, but it is often changed to major, to give it its dominant qualities.

In jazz, you will usually see minor seventh chord, and not minor triads, so the progression (in F minor) would look like this: :

|| Fm7 | % | % | % | Bbm7 | % | Fm7 | % | C7 | % | Fm7 | % || (% means play the same chord as in the last bar)

There are fewer variations on the minor blues than on the major blues. Putting the IVm chord in the second bar, for example, doesn't sound good. But there are standard variations. The most common is adding the bVI7 chord in the 9th bar. You will see this in most jazz minor blues, like Equinox:

|| Cm7 | % | % | % | Fm7 | % | Cm7 | % | Ab7 | G7 | Cm7 | % || The Ab7 leads strongly to the G7 chord, and is mostly comprised of notes from the C minor scale (except the Gb), so it sounds really good.

Another common variation is found in standards like Footprints (hooray for me!). The bVI7 and the V7 chords are substituted for their sub bV counterparts:

||3/4 Cm7 | % | % | % | Fm7 | % | Cm7 | % | D7 | Db7 | Cm7 | % ||

(Footprints is in 3/4 time, hence the time signature.)

Soloing over minor blues
In soloing over these progressions, you'll probably want to use the following modes:

Of course, for the lazy ones among you, just use C aeolian (over C minor blues), and you're pretty set. If you want an even easier life, use the C minor pentatonic scale or the C blues scale. No problem!