Hi Celumbra,
I not really clear what you have in mind with the “Black-Backgound”. A little scene would be nice.
Typically the black backgound that you get from a render, if no other object is creating this background or has a black material or no light at all, can be instantly rendered out with the “Alpha channel” option enables in the “Render Settings”.
Inside of the scene, this black area will no be in front of any other objects. So, I guess your use is for compositing work.
The main idea of an “Alpha” channel is to provide a “mask” for the complete scene. If you need only a single object or a group of objects, the Compositing Tag allows to set “Object Buffer”. These buffers have numbers and this number need to be in the Render Settings Multipass options. The Compositing Tag allows to give many different numbers to the same or the the same number to different object, and of both options mixed. Check out: Render Settings>Save>Alpha Channel
Keep in mind that the “Alpha” channel works with the RGB information in sync. This can cause problems for, e.g., Color correction (later in “Post”), as pixels are not complete on the borders. For this problem, you can use “Straight Alpha”, which renders any visible RGB value complete, but the “Alpha” channel delivers only the transparent value. In that way the RGB will be affected completely while corrected, then they get masked. (In Compositing this is named Pre-Multiplied for standard Alpha). More about that in my Multi-Pass series.
I prefer Straight Alpha most of the time.
A separate “Alpha” is the same as before, only written into an extra file. Sometimes that is easier to handle.
If you attach an example scene, I will look into it.
All the best
Sassi
