thanks for the info JSWolf and theducks. I understand it better now.
I usually design (rather - put together, I really wouldn't call myself a designer) covers that can handle a change in aspect ratio - I've read lots of ebooks and seen a fair few covers distort on different devices so I was a bit careful about that. Thing is though - changing the aspect ration to none didn't get rid of the white border. (shrugs, at least I have a solution, thanks to Turtle91, and luckily the cover was mostly black already!)
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