They most likely did, but it wasn't noticeable. However since it was greyscale, I didn't care as I would guess all values would have darkened the same amount, thus not changing the modeling or anything. I've read other posts with your concerns with various sprays darkening the blues more than other colors, which would be difficult to deal with.
Laura,
I would be interested in hearing of your procedure for mounting pastels. I would guess it would be helpful for others as well, best started as a new thread?
Thanks for your comments,
Holly