Seeing as no one, including me really was sure what you were after from your vague description which can be interpreted in several ways I would say you are obviously wrong that your description was "clear enough" to anyone, but you.
Course, then you knew what you were wanting so it should be obvious to you.
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With your further description I'd say Craig or Ian's would work.
Use Control Image to in Special to create a darker version of your Scene, place them on the Storyboard and use a vertical left to right motion transition such as Smooth Wipe.
If you want it to be dark for a moment before changing and be light for some time after changing use Split to Split each Scene into three segments (stable dark, transition period and stable light).
Make sure whatever durations you choose for the three segments are duplicated on each version.
Place Dark segment #1 on the Storyboard, then Dark segment #2, Light segment #2 and Light segment #3 (discard Dark #3 and Light #1).
Then place the transition between Dark #2 and Light #2 and set the duration to full Scene duration time, Render.