Here is a quick set of screen grab that show what I did.
Please email me at email@example.com if you have any questions.
1. I re-sized the composition of the clip in After Effects Keeping the current aspect ratio and Render it out.
2. Bring the new footage into MochaPro and Track the background. Make sure you turn “Link to Track” to (None)
3. Reshape the xspline to show the black you want to replace
4. Dupicate the xspline and go into Go into the remove tool. Reshape the splines and change the settings like the image below.