The SERVICE_RELAY_SESSION errors are normal and are nothing to worry about. You will see them after any reboot of the BlackBerry Enterprise Server with a handheld that has the wireless turned off or the battery is dead.
The Blocked worker threads could be something else. If you have a remote Exchange server and the PING time is more then 35ms. If the Exchange server is slow/down or if you are having any LAN/WAN issues.
If you only see the Blocked worker threads after a reboot or restarting the controller service then you should be fine it is just taking a while to complete the mailbox rescan for all of the users within the environment.