Watch the Up a Flat 3: Alt. Dom. Sound online guitar lesson by Kenny Wessel from Shades of Jazz
Here is a very useful application for our old buddy, the blues scale. If we build a minor pentatonic scale up a minor third (or flat third, that's 3 frets, guitarists!) from the root of our dominant seventh chord, we get an altered dominant scale that's very handy to use and beautiful sounding. For example, if we're playing over an F7 chord, and we want to introduce some altered harmony, some tensions, we can play an Ab minor pentatonic scale. The notes are Ab, B, Db, Eb, and Gb. Played over an F7 chord, the tensions we get are: #9, #11, b13, b7, and b9. Every note in this scale, with the exception of the b7, is an altered tension! This is a very useful sound and a great way to get quickly into the altered dominant universe, creating more tension, chromaticism and color and bringing some more interest and leading possibilities into our playing. And we're still just playing blues scales (minor pentatonics)!
(Note that if we are using this scale, we should be aware of what the rhythm section is playing. If our chord player, is using an altered dominant chord, any of the altered tensions - b9, #9, #11, b13, this scale choice will be very hip. If they are laying on natural 9s, natural 11s, natural 13s, all non-altered notes, then we might not want to use scale so heavily. We always need to connect with the musicians around us, when making music!)