Elizabeth,
As I (mis)understand it, you have a background part partially covered by and partly showing through a foreground part with different elements.
I believe you can simply apply a certain degree of transparency to (each of the different elements of) the foreground part, through the Transparency panel by setting the Opacity.
To get the general darkening of the background part, you can use black (or black/grey) fill behind the other elements, possibly with a transition such as a gradient/blend/multiple paths.
You can use different transparencies for the different elements, such as higher opacity for the white/pale parts.
This will let the background parts show through to different degrees, which you can adjust until you are satisfied with the appearance.