If you are using an LCD, how black is black? Try this experiment.
If you have an iPhone, turn it off. Go into a pitch black room (I mean PITCH black--no light leaks--totally sealed).
Then, turn on your iPhone. The Apple logo comes up--you can just cover that with your thumb.
The bright gray glow you see emitting from your iPhone? That is "black" on an LCD. But it turns out its not really that black is it?
In fact, its such a bright gray, a fully "black" iPhone display can illuminate a dark room.
This is the problem with using generic LCD's for color correction. Their black level isn't black enough, so your colors get washed out and muddied.
Try it sometime...