Kindle cover problem

Hello everyone! I am new here so not sure if this is correct category.

Anyway...

I am formatting books for living. For print and ebooks. So far I didn't had problems with any of my clients, but...

One client is asking that I make kindle cover with black background.

It is mobi file and cover is uploaded with correct kindle size. So when you open book in kindle device (paperwhite) you need to "slide" to the left to see cover. And that "screen" has left and right white bar, or to say margins. Now, I know how to stretch cover to fill full screen on kindle, but in this case, that is not possible (client don't want it). Now, my question is, is there any way to change "white bars" to black? That is what my client asking. I saw some books have black "bars" on cover screen, but mostly of them are white.

I used different softwares for converting to mobi, and none of them work. Not sure if that is just kindle option and not actually in mobi file

thank you and nice to be here