Black background for transparent video

Is there any way to show the customers a transparent video on a black background?

Yes but I think the unfortunate part is how you have to do it, it makes it less easily usable for the client. You would offer them basically two renders the first being the video on black, the second video being the alpha layer and you make them both in the same file one after the other. Then the client needs to be able to cut that alpha with there own software, and know how to do it. Other than this I can't see a way. You could offer both and group them together as a set and hope they don't buy the separate alpha version if they don't know how to use it.