If I understand correctly, what you’re requesting has already been requested before…
…AND has already been implemented in PhotoLab. From the user guide:
Whether you are auto-cropping or manually cropping, you have the option to straighten the horizon directly, without having to go through the Horizon tool:
Place the mouse next to one of the four corners of the cropping grid and a double curve will appear.
Press the left mouse button and rotate in the desired direction.
Use the grid as reference if you want to straighten the horizon.
As stuck said, use the Auto crop mode to let PL automatically remove the black areas.