The Black BG refers to the slice itself, you can have multiple slices in a Screen in Resolume with specific RGB adjustment options and various overlaps, blends etc.
Fading the slices individually would change the looks, so what makes sense is to fade the output as a whole when using the Composition Master.
I hope this explains a bit why we're doing this.
What you could do to get your outputs to fade to black, would be using a separate layer on top of your content and playing a black solid color there. Then when you have the layer on Alpha blend mode, you could use the layer fader to fade in the black over the content.