Watch the Level 4: Jazz Blues Soloing online guitar lesson by David Hamburger from Take 5: Jazz Blues Soloing
Admittedly, the term "altered chord" seems kind of vague. It's not as specific and literal as "dominant seventh chord", which means a major triad with a flat or minor seventh on top. An altered chord is always a dominant seventh chord, but there are three different tones you could potentially add in to make it altered: a b13, a b9 or a #9. To make things even trickier, it could be some combination of those three notes, although for our purposes it's usually not both the b9 and the #9 just because those are hard to play at the same time on the guitar.