Thanks, as you have noticed, it is the application of the blues scale to improvisation that is bothering me. It sounds like the answer is (more or less) brute force memorisation (ie practice). Then again, using any scale (eg C major on a C harp), I suppose it would be easy to hit a wrong note unless you know exactly which note you want to hit next (and manage to hit it); so the general problem remains the same for any scale, and the solution is the same in all cases: practice hitting the notes you want to hit. I think.