how to let black screen follow item in clip?

Hello!

I thought I'd ask here for help since I can't find a good tutorial that can help me with what I want.

I have this clip where a bus passes by, and by the end of the bus I want a black screen following the back of the bus. So I want it sliding in/ sticking on the back of the bus that goes from the right to the left side of the screen. But I don't know how I can let the black screen 'follow' the bus and eventually fill the whole screen once it has been passed by. 

But I don't know if its hard to answer my question in text, but maybe someone knows a tutorial that can help me or close enough to what I want? 

Thanks in advance,

Christa.