With all the respect, but I think you have that kind of problem, because you do not know much about FCP, not even the basics.
So I guess, first of all, you should post Qs like that in the Basics forum.
To answer your Q: The black is around, because you have a smaller sized clip thrown into a bigger canvas.
Usually, if you drop the first clip into a new Seq, FCP is asking if the settings of the clip should be made the Seq settings, then you just confirm.
Or you press Command + “0” to open the timeline settings and you adjust them manually to your needs.
greetings,
Sascha