Ah. Kind of a catch 22. It won't list the configuration in Tunnelblick's window, so you can't rename it with Tunnelblick.
First, quit Tunnelblick.
Then change the name of the configuration file. The file should be in one of two folders:
/User/YOUR-USERNAME/Library/Application Support/Tunnelblick/Configurations
or
/User/YOUR-USERNAME/Library/openvpn
(the first one is the most likely, so try that first).
Once you find the configuration file (it will end in .ovpn or .conf), change the name so it has only the characters which are allowed. It can't include any of the following characters: "#&;:~|*?'\"~<>^[]{}$%". The only non-printable character that is allowed is the space character.
Then launch Tunnelblick. It will offer to convert the OpenVPN configuration file to a Tunnelblick VPN Configuration. Allow that, and you should be all set.