Premiere Elements on what computer operating system? Have you updated 14 to 14.1 yet using an opened project's Help Menu/Updates.
Now to your immediate question and issue.
I am assuming that you have portrait oriented video imported into the Premiere Elements landscape oriented program/project.
Was the video recorded while you were holding the iPhone in portrait orientation. What is the properties of this video?
The Adobe project presets are for landscape oriented imports.
If you want to get rid of the black borders in the situation that I believe that you are in
a. record the video in landscape
b. create you own special Premiere Elements project preset...see the concepts in the following for what is involved in that type of approach
c. scale the video to fit the Edit area monitor to the point where the black borders are gone...you will lose image in the zoomed in look achieved.
d. the project does have a Crop effect (fx Effects/Video/Transform/Crop and zoom option in the Crop effect edit in Applied Effects Tab/Applied Effects Palette/Crop Panel expanded after you apply the Crop effect to the clip.
Please consider, and then let me know if I have gauged you situation correctly. If not, please give more details. Any questions or need clarification on the above, please do not hesitate to ask.