black and white layer mask changes colour when inverted

[defaultlniisp70nl7n]

Hello, 

I am trying to edit a black and white clip where a bus drives past the camera and as the bus passes, a new scene is revealed (like a swoop shot) but when I select my borders of the bus and click 'inverted', it goes back to its orignal colour clip and not pure black like it normally does. Any help? 

[Ali Jaber]

Can you post a screenshot or more showing the program monitor and effect controls panel?