I think a passing tone is playing a note not ordinarily found in a scale to move to one that is, for color. Or no, it could be a note in a scale that wouldn't sound decent holding for any length of time. Like, not a strong ... i forget all the lingo ... like root, 3rd, 5th, 7th. triad stuff (which means root, 3rd and 5th i think, the basis of a lot of chords). What you gotta do is go buy yourself a basic theory book and learn this crap. suck, but gotta be done.

here's a blues scale:

|-------------------------------------------------------------
|------------------------------------8---10
|--------------7----8---9---
|---7---10
|
|

the 8 on the G string (E flat) is the flatened 5 and is used for color. you'll understand all this when you get down some basic theory.