First, you shoot you content with your blue screen and insert the clip on the timeline.
then you create another track below and insert your background. You then apply the chroma key effect on the first track, and with the eye dropper tool you can select the good color.
Hope this helps,
Nat
John Jackman has written a bunch for DV magazine, moderates one of their forums I think, written a couple of books, and has some of his writting and tips archived online.