If I understand you correctly, then sure you can do that and it's easy. First the simple thing:
Slide 1 is the dark one
<------------------------------Put a long (few sec) transition of the Crossfade blend (A/B) type
Slide 2 is the light one
That's it. The pacing of this is controlled by the time on screen for the slides and the transition time.
Now, you can have some fun here too.
Play with a few of the transition types and see what it looks like. Soft edge dissolve, a wipe or even something like a swirl in might be interesting.
Dick