|If it's a Facebook App, It's most likely Flash that's your problem.|
First try pressing CTRL + F5 to refresh the page. This will force it to re-download the page & app again.
Try right clicking the black box, clicking settings and disabling Hardware acceleration If possible. Or enable it if it's already off.
Try re-installing Flash @ http://get.adobe.com/flashplayer/
You could also try it in a different Web browser, Try using Google Chrome or Firefox if you don't have them.