I record mobile games and have my phone screen, but around the edge of the video it adds a black border which I don't want. Does anyone know how to remove it?
If you are playing a portrait oriented game, as Jim Mack said, something has to be in the empty space, that's where the black pixels come from. Unless...
You can use After Effects here. Duplicate the video clip, take the video behind the front one, which you won't touch, then scale it up so that the sides meet the sides of the composition, and add a gausian blur, or any blur, really, to the back layer. Here is what I'm talking about:
Duplicate the first layer (press Ctrl+D or go to Layer > Duplicate):
Scale up the second layer (open up the layer properties by clicking on the arrow > left beside the layers name or select the layer and hit S to open up scale property only):
Add a Blur Effect to the second layer (Select the Layer and go to Effects > Blur and Sharpen > Gaussian Blur)
Note: In order to render faster, use a Fast Blur and enable repeat edge pixels to blur the entire image.
When you view a portrait-oriented video on a landscape-oriented display, something has to occupy the otherwise unused raster. In this case it's black.
Run the video through the ffmpeg crop filter, the link I gave has the full description with examples.