To be honest, like the others "shadow magic" strikes me very much as a relatively modern concept that has little to do with traditional magic. For that matter, pyrokenisis or hydrokenisis are similarly modern and somewhat debatable.
What I've been able to derive from what others have discussed about shadow magic is that it involves the manipulations of shadows, to some extent, and in this regard it is an aspect that involves influencing the mind of another to give the impression that shadows are moving, light has been darkened, etc. In this way the impression of darkness is projected upon another, or otherwise created more in illusion than truth. After all, shadow is actually the absence of lightwaves, so to cause them to move or alter is essentially to cause that individual to be unable to perceive rays of light.
How successful one can be in this is again subject to a great deal of debate, and often can be confused quite readily with naturally occurring environmental factors that could be logically explained.