I am trying to place a green screen video on top of another video. My issue is everytime I add the Keying menu my background image turns black. What could be causing this issue?
The size of the output image is determined by the image connected to the background.
If you plug in a scaled (or differently sized) image as foreground, it will mess up the sizing of the composite.
In this case your setup is correct but the keying node is filling the frame with a value of black information (0) around the image, and a value of 1 in the alpha channel, creating full opacity.
The way to fix this is making the Black clip be a value that is not 0, so that the alpha channel becomes transparent to that value. Changing the value to 0.00001 will make the transparency come back:
I believe the problem is that the alpha channel you are creating is the size of the image containing the dog, therefore that is the only portion of the image being overlayed. To solve your problem you need to have an alpha channel that is the same size as the background image.