Tested on IE 6 and Mozilla Foxfire 2
Hey Kristie ...
I checked it on the above two browsers and it looked great - no black box. I also checked the source code of the page and javascript of the form and they look fine too.
You might as someone who's getting the error what browser they are using and you should be able to duplicate the problem by getting that same browser and testing it - alternatively, you should be able look at the backend stats and see all the browsers that have reviewed the site. If you find it's a minimally used browser: don't worry about it - help the user upgrade to a more popularly used browser. The rule of thumb I've always used for design is "design it for Netscape - it'll work in IE". And if it's IE 7, I'd not worry too much about it "today". That thing is still kinkier than Larry Flint.
All we can do is design for "the standard". If we tried to design for every browser and individual computer configuration, we'd never finish a site.
Beautiful site!
Kev