I think the issue you’re running into is with the inherent way that Premiere works. The program creates transitions between frames and scenes when you’re editing video.

I think the reason you’re seeing the black box is because there is likely a transition frame in your time line that is added by default. Look at the screenshot below.

Notice there is a black frame in the list. To view this in your own project simply open the Premiere project file and click on the orange, “edit”, tab on the left. You can then left click on that transition and cut it. That should remove the black frame you’re seeing.

Hope this helps.

thanks!