When i crop a clip to get rid of a black edge border, i get very noticeable motion blur upon rendering. When i don't crop it, no motion blur occurs at all. All settings are the same except for the cropping.
You are calling it motion blur but it's probably just a softening of the image cause by the cropping. This happens because the video needs to be resized to fix the new crop to the frame. In your Project Properties, set your Full-resolution rendering quality: Best and it will do a better job of resizing the cropped area.