If you want the mask to 'cut in' you will need to set a keyframe 1 frame before your first keyframe with 'no mask' as its attributes. If you want the mask to creep in make that 'no mask' keyframe earlier than one frame before. If you want the mask to jump in it might be easier to just cut the clip.
I want the mask to apply when the wall finishes so it then begins to play the second clip which will then expand. For some reason even when I set a keyframe at the starting frame, it applies the black to the full clip like in the link i attached