I ran into this problem earlier today when trying to stream something. The chat in procaster is black/blank, and the pop-out chat is blank as well. The chat on the channel's webpage itself works fine.
http://i47.tinypic.com/yh553.jpg
It was working fine just yesterday. I believe the only thing that changed between then and now was the updating of Java.
I've tried uninstalling & reinstalling, updating flash, etc.