In chat configuration, if you change Default CSS into your own, you can adjust that CSS to any colors you like.
If you set that CSS into automatic, and you are using standard joomla/mambo template with default CSS definitions, most likely you will not have to adjust a thing.
If you set chat config to use your own CSS template, then file is in your joomla/mambo template directory:
/.../templates/tmeplate_name/css
Or you can set chat config to use directly some file, you can simply download blastchat CSS file and save it to some directory in your server, modify it and that's it.