When rendering your animation, in the Render Globals window, ensure that the box next to Alpha Channel is checked on (this is found under the renderable objects section). THis will render your animation with an alpha channel, which will allow you to comp your render over another background.

Also, if you wish to change the colour of the background in the render, you can do this by selecting the camera you are rendering through, and going to the sttribute editor. In here scroll down to the environment section, and you can change the background colour from black to whatever you want.

Hope this helps

LisaG