Have you changed the artwork since you were approved by iTunes?
I took a look at your podcast RSS feed and I see this is the artwork that's set to be used by iTunes:
http://borntokickarse.com/wp-content/uploads/2015/12/Logo-iTunes-3K1.pngYou're right in that, this artwork doesn't have a black background. Go to
http://podcastsconnect.apple.com/ and log in with your Apple ID. Click the artwork for your show and then click the "refresh feed" link. It may take up to 24 hours for your iTunes Store listing to update. Keep in mind that current subscribers will get your latest episode as soon as they refresh their subscriptions, regardless of what's being displayed in the iTunes Store.
If that doesn't fix it, you'll need to contact iTunes support and ask them what's going on because it's clearly on their end:
https://itunespodcasts.zendesk.com/hc/en-us