Background images, Full bleed vs inset transparency issues

Hi, I'm trying to let some transparent areas on my background png images (full bleed) to show up as black.  Under section themes, I set the background to black under "Sitewide". I noticed that when I set the background image under "inset" it will let the black show through as I hoped the full bleed option would. Is it possible to enable black backgrounds like this for full bleed backgrounds? It seems anytime the background png is set to full bleed, the background color is always white, rather than black.

I suppose I can always just set the background color black on the original art files, but I'm wondering if the above is possible 😕

Thanks in advanced.

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