Clapton is a master of the blues, and a wealth of information is contained in each solo. A key element I picked up from him is knowing how to play with the 3rd of the I7. In this case we are playing a blues in A, so my I7 chord is A7 spelled AC#EG. Clapton really nails that C# when he is on his A7 chord, this really brightens up the sound quite a bit, especially if you are used to just using the A minor pentatonic scale. Simply put, when playing the A7 chord in your blues add in the C#, on the D7 and E7, leave it out.