Simple Cropping Method
Select the Crop Tool and ensure that the crop parameters are blank

Set your background colour to the colour of the border you want, then simply pull out the crop lines beyond the edge of the picture. Try to ensure that you get the same distance on each side so as to keep the border even. Once you are happy, right click the image and select "crop" to crop the image.
This will leave you with the image, now cropped to the new size, with the empty space filled with the background colour, in this case black.
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