As far as I know, it does not support Blu-Ray discs. You can verify this by contacting support if you have not already not tried to configure backup to use your burner with Blu-Ray discs.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator