Mailing -> New mail results in blank page
RESOLVED:
Answer is in Problem with CiviMail Menu on Joomla 3.4.1 - CiviCRM 4.6.2 

Add to civicrm.settings.php
define('CIVICRM_CIVIMAIL_UI_LEGACY', 1);

That fixed it for now as this apparently won't be an option in 4.7