Watch the Affordable Shuffle Solo online guitar lesson by Jeff McErlain from Essentials: Advanced Blues Soloing
If we analyze an A diminished scale over the A7 chord it gives us the following notes: A-Bb-C-C#-Eb-E-F#-G. As you can see, I mixed sharps and flats for what I think is the easiest way to see the scale. If you look inside, you will see the notes of the A7 chord in the scale itself: A-C#-E-G. The remaining notes give us the tension we're looking for. If we give each of the notes in the scale the number representing how they work against the A7, we would get the following: R-b9-#9-3-b5-5-13-b7. So, when we play an A diminished scale over the A7 chord, we're implying A7 (b9, #9, 13), which is a pretty cool sound.
If that's a bit too theory heavy for you, just start by experimenting with the scale on an A7 to get the sound in your head. That is ultimately what it comes down to, but when dealing with things like the diminished, we really must know how it functions and when to use it properly. I highly suggest listening to Robben Ford's playing to get the sound in your head. John Scofield as well!