What's causing this black square when I convert to another file type?

The image is a banner I made for my Wordpress website that I'm working on. I created it in Illustrrator, then tried converting it to several different file types (jpg, gif, png, pdf) but the black square keeps showing up when I place it on the site. It doesn't show up in the original AI doc. I've tried upgrouping it and isolating the element, but to no avail. Any suggestions?