The xml config file isn’t closed correctly and the parsing of the parameters doesn’t work. There is a method around it:
Edit the file: /home/user/.config/bless/preferences.xml
<pref name="ByteBuffer.TempDir">/tmp</pref> <!-- add this -->
</preferences> <!-- Add this -->
And make that file readonly, this way it won't get modified each time you restart bless.
Note: You won't be able to modify the preferences from the bless editor, you will have to edit the config file by yourself.
Another method is to rebuild Bless, applying a patch: https://bugs.launchpad.net/ubuntu/+source/bless/+bug/1622951