Hi Leo,
Thank you for letting me know it’s browser related. I suspected it might be. But it seemed implausible because the other tabs I have open use transparent backgrounds in the logos, and the logos don’t show up with a black background whereas mine does.
When I started digging, I found the Vanilla post you linked to and saw it mentioned there was a software bug involved: “If Vanilla resizes a PNG with a transparent background, it gets a black background!” Another Stackoverflow thread confirmed the same. So, I wondered if it was related to how WordPress or GP was resizing the favicon for the different display sizes and in doing the resize, caused the transparency/alpha channel data to get lost or compress in such a way that it caused the black background.
I’ll probably do a bit more research and then give up on this by putting a white background behind my transparent logo.
Thanks for your help.