@BerkSmash1984
I use both MMM-PenPlotter and MMM-Wallpaper (the jupadin version) and was able to make the images transparent with no black box. You need to edit each of the svg files in the example_svg folder.

Most of the files were updated by adding

fill="transparent"

in the section with:

<desc>pwidth:500;pheight:500;</desc>
    <path
       stroke="#999999"
       stroke-width="0.2mm"
       fill="transparent"
       

There were a couple where background was already defined in the 3rd row of the files so it had to be updated there. Let me know if you need more specifics.