Hi Martin,
When you say “going to black in the background” my first guess is that you’re talking about the flickering of the image.
If that’s the case, you can apply a simple wiggle expression to the opacity on the layer you would like to apply the effect to.
So…………………
[option] click on the opacity stopwatch for that layer. This will open/enable your opacity expression for that layer.
Type wiggle(12,200) These are my random numbers just for this example. You can change the numbers to tweak your results, but this is a good starting point.
The first number in parentheses is the number of opacity changes per second. The second number is the range that your opacity will change.
My second guess is that you might be asking how to “feather” a layer into a black background.
If that’s the case, you’ll need to read up on masking layers within After Effects. I don’t have any specific links, but I know they’re out there. Unfortunately, it’s a bit much to explain in a reply.
Regards,
Bob